A drug used with rituximab to treat chronic lymphocytic leukemia (CLL) that has come back. It is also used to treat follicular B-cell non-Hodgkin lymphoma (NHL) and small lymphocytic lymphoma (SLL) that have come back after treatment with other anticancer therapy. It is also being studied in the treatment of other types of cancer. Idelalisib blocks certain proteins, which may help keep cancer cells from growing and may kill them. It is a type of kinase inhibitor. Also called Zydelig.