Medical Claims and Benefits Payable - Medical Claims and Future Benefits (Details) - USD ($) $ in Millions	Mar. 31, 2018	Dec. 31, 2017	Mar. 31, 2017	Dec. 31, 2016
Other Liabilities Disclosure [Abstract]
Fee-for-service claims incurred but not paid (IBNP)	$ 1,586	$ 1,717
Pharmacy payable	127	112
Capitation payable	62	67
Other	248	296
Medical claims and benefits payable	$ 2,023	$ 2,192	$ 1,926	$ 1,929