Segment Information - Revenue by Product Type (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Revenue from External Customer [Line Items]
Premiums and fees, mail order pharmacy revenues and other revenues	$ 33,594	$ 31,003	$ 27,931
Medical [Member]
Revenue from External Customer [Line Items]
Premiums and fees, mail order pharmacy revenues and other revenues	24,476	22,933	20,973
Disability [Member]
Revenue from External Customer [Line Items]
Premiums and fees, mail order pharmacy revenues and other revenues	1,767	1,616	1,413
Life Accident And Health [Member]
Revenue from External Customer [Line Items]
Premiums and fees, mail order pharmacy revenues and other revenues	4,739	4,322	3,680
Mail Order Pharmacy [Member]
Revenue from External Customer [Line Items]
Premiums and fees, mail order pharmacy revenues and other revenues	2,239	1,827	1,623
Other Products [Member]
Revenue from External Customer [Line Items]
Premiums and fees, mail order pharmacy revenues and other revenues	$ 373	$ 305	$ 242