Inclusion Criteria:

          -  Diagnosis of PMF, PPV-MF, or PET-MF.

          -  DIPSS risk category of intermediate-1, intermediate-2, or high.

          -  Palpable spleen of ≥ 5 cm below the left costal margin on physical examination at the
             screening visit.

          -  Active symptoms of MF at the screening visit, as demonstrated by the presence of a TSS
             of ≥ 10 using the Screening Symptom Form.

          -  Participants with an ECOG performance status score of 0, 1, or 2.

          -  Screening bone marrow biopsy specimen and pathology report(s) available that was
             obtained within the prior 2 months or willingness to undergo a bone marrow biopsy at
             screening/baseline; willingness to undergo bone marrow biopsy at Week 24 and every 24
             weeks there after. Screening/baseline biopsy specimen must show diagnosis of MF.

          -  Life expectancy of at least 24 weeks.

          -  Willingness to avoid pregnancy or fathering children.

        Exclusion Criteria:

          -  Prior use of any JAK inhibitor.

          -  Prior therapy with any drug that inhibits PI3K (examples of drugs targeting this
             pathway include but are not limited to INCB040093, idelalisib, duvelisib, buparlisib,
             copanlisib, and umbralisib).

          -  Use of experimental drug therapy for MF or any other standard drug (eg, danazol,
             hydroxyurea) used for MF within 3 months of starting study drug and/or lack of
             recovery from all toxicities from previous therapy to ≤ Grade 1.

          -  Inability to swallow food or any condition of the upper gastrointestinal tract that
             precludes administration of oral medications.

          -  Recent history of inadequate bone marrow reserve.

          -  Inadequate liver and renal function at screening.

          -  Active bacterial, fungal, parasitic, or viral infection that requires therapy.

          -  Active HBV or HCV infection that requires treatment or at risk for HBV reactivation.

          -  Known HIV infection.

          -  Uncontrolled, severe, or unstable cardiac disease that in the investigator's opinion
             may jeopardize the safety of the participant or compliance with the Protocol.

          -  Active invasive malignancy over the previous 2 years.

          -  Splenic irradiation within 6 months before receiving the first dose of study drug.

          -  Concurrent use of any prohibited medications.

          -  Active alcohol or drug addiction that would interfere with the ability to comply with
             the study requirements.

          -  Use of any potent CYP3A4 inhibitors or inducers within 14 days or 5 half
             lives(whichever is longer) before the first dose of study drug or anticipated during
             the study.

          -  Inadequate recovery from toxicity and/or complications from a major surgery before
             starting therapy.

          -  Currently breastfeeding or pregnant.

          -  Any condition that would, in the investigator's judgment, interfere with full
             participation in the study, including administration of study drug and attending
             required study visits; pose a significant risk to the participant; or interfere with
             interpretation of study data.

          -  History of Grade 3 or 4 irAEs from prior immunotherapy.

          -  Receipt of any live vaccine within 30 days of the first dose of study drug