Segment Information - Segment Financial Information (Details) - USD ($) $ in Millions	3 Months Ended															12 Months Ended
	Dec. 31, 2016		Sep. 30, 2016		Jun. 30, 2016		Mar. 31, 2016		Dec. 31, 2015		Sep. 30, 2015		Jun. 30, 2015		Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Segment Reporting Information [Line Items]
Premiums																$ 30,626	$ 29,642	$ 27,214
Fees and other revenues																4,760	4,488	4,141
Net investment income																1,147	1,153	1,166
Mail order pharmacy revenues																2,966	2,536	2,239
Total operating revenues																39,499	37,819	34,760
Net realized investment gains (losses)																169	57	154
Total revenues	$ 9,944		$ 9,880		$ 9,960		$ 9,884		$ 9,528		$ 9,389		$ 9,492		$ 9,467	39,668	37,876	34,914
Depreciation and amortization																610	585	588
Total benefits and expenses																36,689	34,549	31,610
Income before taxes	605		742		813		819		659		878		936		854	2,979	3,327	3,304
Income taxes and net income attributable to noncontrolling interests																1,112	1,233	1,202
Shareholders' Net Income	$ 382	[1],[2]	$ 456	[1],[3]	$ 510	[1]	$ 519	[1]	$ 426	[1]	$ 547	[1]	$ 588	[4]	$ 533	1,867	2,094	2,102
Operating Segments [Member] | Global Health Care [Member]
Segment Reporting Information [Line Items]
Premiums																23,295	22,696	20,709
Fees and other revenues																4,623	4,357	4,005
Net investment income																315	340	337
Mail order pharmacy revenues																2,966	2,536	2,239
Total operating revenues																31,199	29,929	27,290
Net realized investment gains (losses)																119	43	79
Total revenues																31,318	29,972	27,369
Depreciation and amortization																526	526	513
Total benefits and expenses																28,467	27,028	24,610
Income before taxes																2,851	2,944	2,759
Income taxes and net income attributable to noncontrolling interests																1,100	1,150	1,059
Shareholders' Net Income																1,751	1,794	1,700
Operating Segments [Member] | Global Supplemental Benefits [Member]
Segment Reporting Information [Line Items]
Premiums																3,226	3,000	2,844
Fees and other revenues																49	46	52
Net investment income																110	103	109
Mail order pharmacy revenues																0	0	0
Total operating revenues																3,385	3,149	3,005
Net realized investment gains (losses)																(5)	0	3
Total revenues																3,380	3,149	3,008
Depreciation and amortization																54	31	50
Total benefits and expenses																3,052	2,849	2,734
Income before taxes																328	300	274
Income taxes and net income attributable to noncontrolling interests																60	33	41
Shareholders' Net Income																268	267	233
Operating Segments [Member] | Group Disability And Life [Member]
Segment Reporting Information [Line Items]
Premiums																4,002	3,843	3,549
Fees and other revenues																98	91	86
Net investment income																343	337	335
Mail order pharmacy revenues																0	0	0
Total operating revenues																4,443	4,271	3,970
Net realized investment gains (losses)																59	5	22
Total revenues																4,502	4,276	3,992
Depreciation and amortization																28	26	22
Total benefits and expenses																4,273	3,796	3,513
Income before taxes																229	480	479
Income taxes and net income attributable to noncontrolling interests																65	152	148
Shareholders' Net Income																164	328	331
Operating Segments [Member] | Other Operations Segment [Member]
Segment Reporting Information [Line Items]
Premiums																103	103	112
Fees and other revenues																11	13	14
Net investment income																358	369	384
Mail order pharmacy revenues																0	0	0
Total operating revenues																472	485	510
Net realized investment gains (losses)																(5)	9	15
Total revenues																467	494	525
Depreciation and amortization																1	1	2
Total benefits and expenses																369	374	413
Income before taxes																98	120	112
Income taxes and net income attributable to noncontrolling interests																30	40	33
Shareholders' Net Income																68	80	79
Corporate [Member]
Segment Reporting Information [Line Items]
Premiums																0	0	0
Fees and other revenues																(21)	(19)	(16)
Net investment income																21	4	1
Mail order pharmacy revenues																0	0	0
Total operating revenues																0	(15)	(15)
Net realized investment gains (losses)																1	0	35
Total revenues																1	(15)	20
Depreciation and amortization																1	1	1
Total benefits and expenses																528	502	340
Income before taxes																(527)	(517)	(320)
Income taxes and net income attributable to noncontrolling interests																(143)	(142)	(79)
Shareholders' Net Income																$ (384)	$ (375)	$ (241)

[1] Shareholders’ net income includes after-tax charges related to the Company’s proposed merger with Anthem as follows: $29 million in the third quarter of 2015, $28 million in the fourth quarter of 2015, $36 million in the first quarter of 2016, $26 million in the second quarter of 2016, $46 million in the third quarter of 2016 and $39 million in the fourth quarter of 2016. See Note 3 to the Consolidated Financial Statements for additional details.

[2] Shareholders’ net income includes an after-tax charge of $80 million for an allowance for the risk corridor receivable balance in the fourth quarter of 2016. See Note 22 to the Consolidated Financial Statements for additional details.

[3] Shareholders’ net income includes an after-tax charge of $25 million for charges related to litigation matters in the third quarter of 2016. See Note 21 to the Consolidated Financial Statements for a description of litigation matters.

[4] Shareholders’ net income includes an after-tax charge of $65 million for the early extinguishment of debt in the second quarter of 2015. See Note 5 to the Consolidated Financial Statements for additional details.