Variable Interest Entities (VIEs) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016	Dec. 31, 2016
Variable Interest Entity [Line Items]
Administrative fee revenue	$ 13	$ 14
Variable Interest Entity, Primary Beneficiary
Variable Interest Entity [Line Items]
Amount paid for health care services	31	$ 31
Total carrying amount of assets	17		$ 18
Total amount of liabilities	$ 17		$ 18