Segment Information - Revenues by Product Type (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Revenue from External Customer [Line Items]
Premiums	$ 32,307	$ 30,626	$ 29,642
Mail order pharmacy revenues	2,979	2,966	2,536
Other	338	378	374
Revenues from external customers	40,153	38,352	36,666
Medical [Member]
Revenue from External Customer [Line Items]
Premiums	24,538	23,295	22,696
Fees	4,503	4,368	4,107
Total premiums and fees	29,041	27,663	26,803
Guaranteed Cost [Member]
Revenue from External Customer [Line Items]
Premiums	6,245	4,610	4,761
Experience Rated [Member]
Revenue from External Customer [Line Items]
Premiums	2,741	2,383	2,329
Stop Loss [Member]
Revenue from External Customer [Line Items]
Premiums	3,483	3,082	2,701
International Health Care [Member]
Revenue from External Customer [Line Items]
Premiums	1,934	1,859	1,834
Dental [Member]
Revenue from External Customer [Line Items]
Premiums	1,791	1,586	1,392
Medicare [Member]
Revenue from External Customer [Line Items]
Premiums	5,534	6,621	6,142
Medicaid [Member]
Revenue from External Customer [Line Items]
Premiums	1,061	1,146	1,102
Medicare Part D [Member]
Revenue from External Customer [Line Items]
Premiums	764	1,122	1,589
Other Medical [Member]
Revenue from External Customer [Line Items]
Premiums	985	886	846
Disability [Member]
Revenue from External Customer [Line Items]
Total premiums and fees	2,091	2,045	1,899
Life, Accident and Supplemental Health [Member]
Revenue from External Customer [Line Items]
Total premiums and fees	$ 5,704	$ 5,300	$ 5,054