Inclusion Criteria:

          -  Age ≥ 18 years, ECOG performance status ≤ 2

          -  Histologically confirmed diagnosis of iNHL (Subtypes include FL Grades 1 to 3a,
             marginal zone lymphoma (splenic, nodal, or extranodal), or SLL

          -  Must have received 1 prior systemic regimen for iNHL

          -  Must have documented radiologic evidence of disease progression, and at least 1
             bi-dimensionally measurable lesion ≥ 1.5 cm (which has not been previously
             irradiated), according to 2007 revised IWG criteria

          -  Must have adequate organ function defined by the following laboratory parameters:

               -  Absolute neutrophil count (ANC) ≥ 1.0 × 10^9/L

               -  Platelet count ≥ 75 × 10^9/L

               -  Serum creatinine < 2.0 mg/dL (197 µmol/L)

               -  Total bilirubin ≤ 1.5 × upper limit of normal (ULN) (exception: subjects with
                  Gilbert's Syndrome may have a bilirubin > 1.5 × ULN)

               -  Aspartate transaminase (AST)/serum glutamic-oxaloacetic transaminase (SGOT) and
                  alanine aminotransferase (ALT)/serum pyruvic transaminase (SGPT) ≤ 3.0 × ULN

        Exclusion Criteria:

          -  Anticancer treatment, major surgery, or use of any investigational drug within 28 days
             before the start of study intervention; palliative radiation therapy is allowed if > 7
             days and any toxicity is Grade ≤ 1

          -  Clinical or histological evidence of transformation to a more aggressive subtype of
             lymphoma or grade 3b FL or Richters' transformation or CLL

          -  Prior allogeneic hematopoietic stem cell transplant (HSCT); treatment with a PI3K
             inhibitor

          -  History of drug-induced colitis or pneumonitis; TB treatment ≤ 2 years prior to
             randomization; administration of a live or live attenuated vaccine within 6 weeks of
             randomization

          -  Ongoing treatment with chronic immunosuppressants or systemic steroids or treatment
             for systemic bacterial, fungal, or viral infection

          -  Active cytomegalovirus (CMV) or Epstein-Barr virus (EBV) infection

          -  Unable to receive prophylactic treatment for pneumocystis, herpes simplex virus (HSV),
             or herpes zoster (VZV) at screening

          -  Concurrent administration of medications or foods that are strong inhibitors or
             inducers of cytochrome P450 3A (CYP3A). No prior use within 2 weeks before the start
             of study intervention.

          -  Baseline QTcF > 500 ms

          -  Concurrent active malignancy other than non-melanoma skin cancer or carcinoma in situ
             of the cervix, bladder cancer, or prostate cancer not requiring treatment. Subjects
             with previous malignancies are eligible if they have been disease-free for 2 years or
             more.

          -  Unstable or severe uncontrolled medical condition that would, in the Investigator's
             judgment, increase the subject's risk to participating in this study.