Segment Information - Revenues by Product Type (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Revenue from External Customer [Line Items]
Premiums	$ 8,030	$ 7,605	$ 24,143	$ 23,005
Mail order pharmacy revenues	733	762	2,200	2,207
Revenues from external customers	$ 9,967	$ 9,523	$ 29,962	$ 28,766