Segment Information - Revenue by Geographical Area (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Revenues from External Customers and Long-Lived Assets [Line Items]
Premiums and fees, mail order pharmacy revenues and other revenues	$ 33,594	$ 31,003	$ 27,931
United States
Revenues from External Customers and Long-Lived Assets [Line Items]
Premiums and fees, mail order pharmacy revenues and other revenues	30,070	27,868	25,217
Foreign [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Premiums and fees, mail order pharmacy revenues and other revenues	$ 3,524	$ 3,135	$ 2,714