Medical Claims and Benefits Payable - Medical Claims and Future Benefits (Details) - USD ($) $ in Millions	Mar. 31, 2017	Dec. 31, 2016	Mar. 31, 2016	Dec. 31, 2015
Other Liabilities Disclosure [Abstract]
Fee-for-service claims incurred but not paid (IBNP)	$ 1,425	$ 1,352
Pharmacy payable	133	112
Capitation payable	36	37
Other	332	428
Medical claims and benefits payable	$ 1,926	$ 1,929	$ 1,940	$ 1,685