Segment Information - Revenue by Geographical Area (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Revenues from External Customers and Long-Lived Assets [Line Items]
Premiums and fees, mail order pharmacy revenues and other revenues	$ 9,094	$ 8,442	$ 27,386	$ 24,993