Inclusion Criteria:

          -  Confirmed diagnosis of CLL/SLL requiring therapy as defined by iwCLL 2018 criteria

          -  Previously treated with a covalent BTK inhibitor

          -  Eastern Cooperative Oncology Group (ECOG) 0-2

          -  Absolute neutrophil count ≥ 0.75 × 109/L without granulocyte-colony stimulating factor
             support

          -  Hemoglobin ≥ 8 g/dL not requiring transfusion support or growth factors within 14 days
             of Cycle 1 Day 1

          -  Platelets ≥ 50 × 109/L not requiring transfusion support or growth factors within 14
             days of C1D1. If an investigator has chosen bendamustine/rituximab as the Arm B
             treatment, platelets must be ≥ (75 × 109/L).

          -  AST and ALT ≤ 3.0 x upper limit of normal (ULN).

          -  Total bilirubin ≤ 1.5 x ULN.

          -  Estimated creatinine clearance of ≥ 30 mL/min.

        Exclusion Criteria:

          -  Known or suspected Richter's transformation at any time preceding enrollment.

          -  Known or suspected history of central nervous system (CNS) involvement by CLL/SLL

          -  Ongoing drug-induced liver injury

          -  Active uncontrolled auto-immune cytopenia

          -  Significant cardiovascular disease

          -  History of allogeneic or stem cell transplantation (SCT) or chimeric antigen
             receptor-modified T cells (CAR-T) therapy within the past 60 days

          -  Active hepatitis B or hepatitis C

          -  Known active cytomegalovirus (CMV) infection.

          -  Active uncontrolled systemic bacterial, viral, fungal or parasitic infection.

          -  Known Human Immunodeficiency Virus (HIV) infection, regardless of CD4 count.

          -  Clinically significant active malabsorption syndrome or inflammatory bowel disease

          -  Prior exposure to non-covalent (reversible) BTK inhibitor.

          -  Patients requiring therapeutic anticoagulation with warfarin or another Vitamin K
             antagonist.

          -  Current treatment with strong cytochrome P450 (CYP) 3A4 (CYP3A4) inhibitors or
             inducers and/or strong P-glycoprotein (P-gp) inhibitors

          -  Vaccination with a live vaccine within 28 days prior to randomization

          -  Patients with the following hypersensitivity:

               1. Known hypersensitivity, including anaphylaxis, to any component or excipient of
                  LOXO-305, idelalisib, and bendamustine

               2. Prior significant hypersensitivity to rituximab