Study to Evaluate Safety, Tolerability, and Pharmacokinetics of Idelalisib in Adults Receiving Ruxolitinib as Therapy for Primary, Post-Polycythemia Vera, or Post-Essential Thrombocythemia Myelofibrosis With Progressive or Relapsed Disease

Key Inclusion Criteria: * Individuals must have been on a stable dose of ruxolitinib for at least 4 weeks prior to study entry * Individuals with PMF, post-PV MF, or post-ET MF classified as high risk or intermediate risk as defined by the Dynamic International Prognostic Scoring System (DIPSS) for PMF or DIPSS Plus, if cytogenetics are available * Individuals with PMF, post-PV MF, or post-ET MF who are receiving ruxolitinib and meet 2013 Revised International Working Group for Myelofibrosis Research and Treatment (IWG-MRT) and European Leukemia Net (ELN) response criteria with progressive and relapsed disease, with modifications for progressive disease complete remission (CR), partial remission (PR), or clinical improvement (CI) * European Cooperative Oncology Group (ECOG) performance status of ≤ 2 * Required screening laboratory values as described in the protocol * Willing and able to comply with scheduled visits, drug administration plan, imaging studies, laboratory tests, other study procedures, and study restrictions including mandatory prophylaxis for pneumocystis jiroveci pneumonia (PJP) * Able to understand and willing to sign the informed consent form Key Exclusion Criteria: * Individuals on a stable ruxolitinib dose of 5 mg once daily * History of prior allogeneic bone marrow progenitor cell or solid organ transplantation * Ongoing drug-induced liver injury, alcoholic liver disease, non-alcoholic steatohepatitis, primary biliary cirrhosis, extrahepatic obstruction caused by cholelithiasis, cirrhosis of the liver * Ongoing drug-induced pneumonitis * Ongoing inflammatory bowel disease * Ongoing alcohol or drug addiction * Symptomatic congestive heart failure (New York Heart Association Classification \> Class II), unstable angina, or unstable cardiac arrhythmia requiring medication * Known hypersensitivity to the study investigational medicinal product (IMP), the metabolites, or formulation excipients * Unwilling or unable to take oral medication * Unresolved non-hematologic toxicities from prior therapies that are \> Common terminology Criteria for Adverse Events (CTCAE) Grade 1 (with the exception of alopecia \[Grade 1 or 2 permitted\]) * Pregnant or lactating females * Cytomegalovirus (CMV): Ongoing infection, treatment, or prophylaxis within the past 28 days NOTE: Other protocol defined Inclusion/ Exclusion criteria may apply.