This Phase 3, randomized, double-blind, placebo-controlled study is to evaluate the effect of idelalisib in combination with rituximab on the onset, magnitude, and duration of tumor control in participants previously treated for chronic lymphocytic leukemia (CLL). Eligible patients will be randomized with a 1:1 ratio into 1 of the 2 treatment arms to receive either idelalisib plus rituximab or placebo plus rituximab. Participants who are tolerating primary study therapy but experience definitive CLL progression are eligible to receive active idelalisib therapy in the extension study, GS-US-312-0117.