Segment Information - Segment Financial Information (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended											12 Months Ended
	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013		Sep. 30, 2013	Jun. 30, 2013		Mar. 31, 2013		Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Segment Reporting Information [Line Items]
Premiums												$ 27,214	$ 25,575	$ 23,017
Fees and other revenues												4,141	3,601	3,291
Mail order pharmacy revenues												2,239	1,827	1,623
Net investment income												1,166	1,164	1,144
Realized investment gains (losses)												154	213	44
Total revenues	8,928	8,757	8,733	8,496	8,151		8,066	7,980		8,183		34,914	32,380	29,119
Depreciation and amortization												588	597	560
Income taxes												1,210	698	853
Segment earnings (loss)												1,996	1,335	1,592
Shareholders' net income	467	534	573	528	361	[1]	553	505	[2]	57	[3]	2,102	1,476	1,623
Realized Investment Gains
Realized investment gains (losses)												154	213	44
Less income taxes (benefits)												48	72	13
Net realized investment gains (losses)												106	141	31
Operating Segments [Member] | Global Health Care [Member]
Segment Reporting Information [Line Items]
Premiums												20,709	19,626	17,877
Fees and other revenues												4,005	3,518	3,321
Mail order pharmacy revenues												2,239	1,827	1,623
Net investment income												337	325	259
Segment revenues												27,290	25,296	23,080
Depreciation and amortization												513	529	516
Income taxes												1,035	822	793
Segment earnings (loss)												1,646	1,517	1,418
Operating Segments [Member] | Global Health Care [Member] | Guaranteed Cost [Member]
Segment Reporting Information [Line Items]
Premiums												4,600	4,463	4,256
Operating Segments [Member] | Global Health Care [Member] | Experience Rated [Member]
Segment Reporting Information [Line Items]
Premiums												2,322	2,292	2,022
Operating Segments [Member] | Global Health Care [Member] | Stop Loss [Member]
Segment Reporting Information [Line Items]
Premiums												2,318	1,907	1,672
Operating Segments [Member] | Global Health Care [Member] | International Health Care [Member]
Segment Reporting Information [Line Items]
Premiums												1,827	1,752	1,648
Operating Segments [Member] | Global Health Care [Member] | Dental [Member]
Segment Reporting Information [Line Items]
Premiums												1,257	1,139	1,005
Operating Segments [Member] | Global Health Care [Member] | Medicare [Member]
Segment Reporting Information [Line Items]
Premiums												5,660	5,639	4,969
Operating Segments [Member] | Global Health Care [Member] | Medicaid [Member]
Segment Reporting Information [Line Items]
Premiums												515	317	207
Operating Segments [Member] | Global Health Care [Member] | Medicare Part D [Member]
Segment Reporting Information [Line Items]
Premiums												1,405	1,387	1,421
Operating Segments [Member] | Global Health Care [Member] | Other [Member]
Segment Reporting Information [Line Items]
Premiums												805	730	677
Operating Segments [Member] | Global Supplemental Benefits [Member]
Segment Reporting Information [Line Items]
Premiums												2,844	2,496	1,975
Fees and other revenues												52	43	30
Net investment income												109	100	90
Segment revenues												3,005	2,639	2,095
Depreciation and amortization												50	50	29
Income taxes												48	50	36
Equity in income of investees												18	17	10
Segment earnings (loss)												230	175	142
Operating Segments [Member] | Group Disability And Life [Member]
Segment Reporting Information [Line Items]
Premiums												3,549	3,348	3,044
Fees and other revenues												86	78	65
Net investment income												335	321	300
Segment revenues												3,970	3,747	3,409
Depreciation and amortization												22	14	11
Income taxes												140	101	116
Segment earnings (loss)												317	259	279
Operating Segments [Member] | Group Disability And Life [Member] | Life [Member]
Segment Reporting Information [Line Items]
Premiums												1,629	1,552	1,426
Operating Segments [Member] | Group Disability And Life [Member] | Disability [Member]
Segment Reporting Information [Line Items]
Premiums												1,681	1,539	1,348
Operating Segments [Member] | Group Disability And Life [Member] | Other [Member]
Segment Reporting Information [Line Items]
Premiums												239	257	270
Operating Segments [Member] | Other Operations Segment [Member]
Segment Reporting Information [Line Items]
Premiums												112	105	121
Fees and other revenues												14	(24)	(101)
Net investment income												384	408	490
Segment revenues												510	489	510
Depreciation and amortization												2	1	2
Income taxes												29	(237)	43
Segment earnings (loss)												68	(394)	82
Corporate [Member]
Segment Reporting Information [Line Items]
Fees and other revenues												(16)	(14)	(24)
Net investment income												1	10	5
Segment revenues												(15)	(4)	(19)
Depreciation and amortization												1	3	3
Income taxes												(90)	(110)	(148)
Segment earnings (loss)												$ (265)	$ (222)	$ (329)

[1] The fourth quarter of 2013 includes an after-tax charge of $40 million for an organizational efficiency plan.

[2] The second quarter of 2013 includes an after-tax charge of $24 million for the Pharmacy Benefits Manager (“PBM”) partnering agreement with Catamaran.

[3] The first quarter of 2013 includes an after-tax gain of $25 million for the GMIB business, an after-tax charge of $507 million for the transaction with Berkshire to effectively exit the Run-off Reinsurance business, and an after-tax charge of $51 million related to the disability claims regulatory matter in the Group Disability and Life segment.