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 antibo…