Study to Evaluate the Efficacy and Safety of Loncastuximab Tesirine Versus Idelalisib in Participants With Relapsed or Refractory Follicular Lymphoma

Inclusion Criteria: * Written informed consent must be obtained prior to any study procedures. * Male or female participants aged 18 years or older, with pathologic diagnosis of follicular lymphoma (FL) (Grade 1, 2, 3A) in the most recent tumor biopsy. * Relapsed or refractory disease following two or more treatment regimens, at least one of which must have contained an anti-CD20 therapy. * Participants who have received previous CD19-directed therapy must have a biopsy which shows CD19 expression after completion of the CD19-directed therapy. * Measurable disease as defined by the 2014 Lugano Classification as assessed by positron emission tomography - computed tomography (PET-CT) or, if not Fluorodeoxyglucose (FDG) avid, CT or magnetic resonance imaging (MRI). * Availability of formalin-fixed paraffin-embedded (FFPE) tumor tissue block (or minimum 10 freshly cut unstained slides if block is not available). Note: Any biopsy since initial diagnosis is acceptable, but if several samples are available, the most recent sample is preferred. * Eastern Cooperative Oncology Group (ECOG) performance status 0 to 2. * Adequate organ function as defined by screening laboratory values within the following parameters: 1. Absolute neutrophil count (ANC) ≥1.0 × 10\^3/μL (off growth factors at least 72 hours), 2. Platelet count ≥75 × 10\^3/μL without transfusion in the past 2 weeks, 3. Alanine aminotransferase, AST, and GGT ≤2.5 × the upper limit of normal (ULN), 4. Total bilirubin ≤1.5 × ULN (participants with known Gilbert's syndrome may have a total bilirubin up to ≤3 × ULN), 5. Calculated creatinine clearance ≥30 mL/min by the Cockcroft and Gault equation. Note: A laboratory assessment may be repeated a maximum of two times during the Screening period to confirm eligibility * Women of childbearing potential (WOCBP)(1) must agree to use a highly effective method(2) of contraception from the time of giving informed consent until at least 9 months after the last dose of study treatment. Men with female partners who are of childbearing potential must agree to use a condom when sexually active or practice total abstinence from the time of giving informed consent until at least 6 months after the participant receives his last dose of study treatment. 1. WOCBP are defined as sexually mature women who have not undergone bilateral tubal ligation, bilateral oophorectomy, or hysterectomy; or who have not been postmenopausal. A postmenopausal state is defined as no menses for 12 months without an alternative medical cause. A high follicle stimulating hormone (FSH) level in the postmenopausal range may be used to confirm a postmenopausal state in women not using hormonal contraception or hormonal replacement therapy. However, in the absence of 12 months of amenorrhea, a single FSH measurement is insufficient. 2. Highly effective forms of birth control are methods that achieve a failure rate of less than 1% per year when used consistently and correctly. Highly effective forms of birth control include hormonal contraceptives associated with inhibition of ovulation (oral, injectable, patch, intrauterine devices), male partner sterilization, or total abstinence from heterosexual intercourse, when this is the preferred and usual lifestyle of the participant. Note: The double-barrier method (e.g., synthetic condoms, diaphragm, or cervical cap with spermicidal foam, cream, or gel), periodic abstinence (such as calendar, symptothermal, post ovulation), withdrawal (coitus interruptus), lactational amenorrhea method, and spermicide-only are not acceptable as highly effective methods of contraception. Exclusion Criteria: * Previous treatment with loncastuximab tesirine. * Previous treatment with idelalisib. * History of hypersensitivity to any of the excipients of loncastuximab tesirine or idelalisib. * Follicular lymphoma which has transformed to diffuse large B-cell lymphoma (DLBCL) or other aggressive lymphomas. * Requires treatment or prophylaxis with a strong cytochrome P450 (CYP) 3A inhibitor, inducer, or sensitive substrate. * History of or ongoing drug-induced pneumonitis. * History of or ongoing inflammatory bowel disease. * Any condition that could interfere with the absorption or metabolism of idelalisib including malabsorption syndrome, disease significantly affecting gastrointestinal function, or resection of the stomach or small bowel. * Active second primary malignancy other than non-melanoma skin cancers, nonmetastatic prostate cancer, in situ cervical cancer, ductal or lobular carcinoma in situ of the breast, or other malignancy that the Sponsor's medical monitor and Investigator agree and document should not be exclusionary. * Autologous transplant within 30 days prior to start of study treatment (C1D1). * Allogenic transplant within 60 days prior to start of study treatment (C1D1). * Active graft-versus-host disease. * Post-transplantation lymphoproliferative disorders. * Human immunodeficiency virus (HIV) seropositive with any of the following: 1. CD4+ T-cell counts \<350 cells/μL. 2. Acquired immuno-deficiency syndrome (AIDS)-defining opportunistic infection within 12 months prior to screening. 3. Not on anti-retroviral therapy, or on anti-retroviral therapy for \< 4 weeks at the time of screening. 4. HIV viral load ≥400 copies/mL. * Serologic evidence of chronic hepatitis B infection and unable or unwilling to receive standard prophylactic anti-viral therapy or with detectable hepatitis B virus (HBV) viral load. * Serologic evidence of hepatitis C infection without completion of curative treatment or with detectable hepatitis C virus (HCV) viral load. * History of Stevens-Johnson syndrome or toxic epidermal necrolysis. * Lymphoma with active central nervous system involvement, including leptomeningeal disease. * Clinically significant third space fluid accumulation (i.e., ascites requiring drainage or pleural effusion that is either requiring drainage or associated with shortness of breath). * Breastfeeding or pregnant. * Significant medical comorbidities, including but not limited to, uncontrolled hypertension (BP ≥160/100 mm Hg repeatedly), unstable angina, congestive heart failure (greater than New York Heart Association class II), electrocardiographic evidence of acute ischemia, coronary angioplasty or myocardial infarction within 6 months prior to screening, uncontrolled atrial or ventricular cardiac arrhythmia, poorly controlled diabetes, or severe chronic pulmonary disease. * Any Grade ≥3 active infection which requires IV antibiotics, IV antiviral, or IV antifungal treatment. * Major surgery, radiotherapy, chemotherapy or other anti-neoplastic therapy within 14 days prior to start of study treatment (C1D1), except shorter if approved by the Sponsor. * Use of any other experimental medication within 30 days prior to start of study treatment (C1D1). * Live vaccine administration within 4 weeks prior to Cycle(C) 1 Day (D) 1. * Failure to recover to ≤ Grade 1 (Common Terminology Criteria for Adverse Events \[CTCAE\] version 5.0) from acute non-hematologic toxicity (except ≤Grade 2 neuropathy or alopecia) due to previous therapy prior to screening. * Any other significant medical illness, abnormality, or condition that would, in the Investigator's judgment, make the participant inappropriate for study participation or put the participant at risk.