Segment Information - Revenues by Product Type (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Revenue from External Customer [Line Items]
Premiums	$ 7,605	$ 7,347	$ 23,005	$ 22,181
Mail order pharmacy revenues	762	643	2,207	1,846
Revenues from external customers	$ 9,523	$ 9,094	$ 28,766	$ 27,386