A Study of Obinutuzumab in Combination With Idasanutlin and Venetoclax in Participants With Relapsed or Refractory (R/R) Follicular Lymphoma (FL) or Rituximab in Combination With Idasanutlin and Venetoclax in Participants With R/R Diffuse Large B-Cell Lymphoma (DLBCL)

Inclusion Criteria:

* Eastern Cooperative Oncology Group (ECOG) performance status of 0, 1, or 2
* B-cell lymphoma classified as either of the following: R/R FL after treatment with at least one prior chemoimmunotherapy regimen that included an anti-cluster of differentiation 20 (CD20) monoclonal antibody; R/R DLBCL after treatment with at least one prior chemoimmunotherapy regimen that included an anti-CD20 monoclonal antibody in participants who are not eligible for second line combination chemotherapy and autologous stem-cell transplantation, have failed second line combination chemotherapy, or experienced disease progression following autologous stem-cell transplantation
* Histologically documented CD20-positive lymphoma
* Fluorodeoxyglucose (FDG)-avid lymphoma (that is \[i.e.\], PET-positive lymphoma)
* At least one bi-dimensionally measurable lesion (greater than \[\>\] 1.5 centimeters \[cm\] in its largest dimension by CT scan or magnetic resonance imaging \[MRI\])
* Availability of a representative tumor specimen and the corresponding pathology report for retrospective central confirmation of the diagnosis of FL or DLBCL

Exclusion Criteria:

* Known CD20-negative status at relapse or progression
* Prior allogeneic stem-cell transplantation (SCT)
* Completion of autologous SCT within 100 days prior to Day 1 of Cycle 1
* Prior standard or investigational anti-cancer therapy as specified: Radioimmunoconjugate within 12 weeks prior to Day 1 of Cycle 1; Monoclonal antibody or antibody-drug conjugate within 4 weeks prior to Day 1 of Cycle 1; Radiotherapy, chemotherapy, hormonal therapy, or targeted small-molecule therapy within 2 weeks prior to Day 1 of Cycle 1
* Clinically significant toxicity (other than alopecia) from prior therapy that has not resolved to Grade less than or equal to (\</=) 2 (according to National Cancer Institute Common Terminology Criteria for Adverse Events \[NCI CTCAE\] version 4.0) prior to Day 1 of Cycle 1
* Grade 3b FL
* History of transformation of indolent disease to DLBCL (expansion-phase only)
* Central nervous system lymphoma or leptomeningeal infiltration
* Treatment with systemic corticosteroids \>20 mg/day, prednisone or equivalent
* Clinical conditions requiring treatment with oral or parenteral anticoagulants or antiplatelet agents unless treatment can be discontinued 7 days (or 5 half-lives) prior to initiation of study treatment (except used as flushes for indwelling catheters)
* History of severe allergic or anaphylactic reaction to humanized or murine monoclonal antibodies
* Known hypersensitivity or allergy to murine products or any component of the obinutuzumab, rituximab, idasanutlin, or venetoclax formulation
* Current or history of hepatitis B virus (HBV) or hepatitis C virus (HCV) infection: positive for hepatitis B surface antigen (HBsAg), total hepatitis B core antibody (HBcAb), or HCV antibody at screening
* History of progressive multifocal leukoencephalopathy (PML)
* History of other malignancy that could affect compliance with the protocol or interpretation of results
* Evidence of any significant, uncontrolled concomitant disease that could affect compliance with the protocol or interpretation of results
* Non-malignant medical illnesses that are uncontrolled or whose control may be jeopardized by study treatment, such as severe hereditary coagulation disorders or insulin-dependent diabetes mellitus that is not optimally controlled with medical management (example, presence of ketoacidosis)
* Major surgical procedure other than for diagnosis within 28 days prior to Day 1 of Cycle 1, or anticipation of a major surgical procedure during the study
* Inadequate hematologic function (unless due to underlying lymphoma), defined as follows: Hemoglobin less than (\<) 9 grams per decilitre (g/dL), absolute neutrophil count (ANC) \<1.5\*10\^9 cells per liter (cells/L), platelet count \<75\*10\^9 cells/L
* Any of the following abnormal laboratory values (unless due to underlying lymphoma): International normalized ratio (INR) or prothrombin time (PT) \>1.5\*upper limit of normal (ULN) in the absence of therapeutic anticoagulation; partial thromboplastin time (PTT) or activated partial thromboplastin time (aPTT) \>1.5\*ULN in the absence of a lupus anticoagulant
* Life expectancy \<3 months