Segment Information - Summarized Segment Financial Information (Details) - USD ($) $ in Millions	1 Months Ended	3 Months Ended								12 Months Ended
	Apr. 30, 2015	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Segment Reporting Information [Line Items]
Premiums										$ 32,307	$ 30,626	$ 29,642
Fees and other revenues										4,867	4,760	4,488
Net investment income										1,226	1,147	1,153
Mail order pharmacy revenues										2,979	2,966	2,536
Total operating revenues										41,379	39,499	37,819
Net realized investment gains (losses)										237	169	57
Total revenues		$ 10,531	$ 10,382	$ 10,318	$ 10,385	$ 9,944	$ 9,880	$ 9,960	$ 9,884	41,616	39,668	37,876
Depreciation and amortization										566	610	585
Total benefits and expenses										38,010	36,689	34,549
Income (loss) before income taxes		758	824	1,134	890	605	742	813	819	3,606	2,979	3,327
Income taxes (benefits) and net (income) loss attributable to noncontrolling interests										1,369	1,112	1,233
Shareholders' Net Income		266	560	813	598	382	456	510	519	2,237	1,867	2,094
After-tax adjustments to reconcile to adjusted income from operations:
Realized investment (gains) losses										(156)	(109)	(40)
Amortization of other acquired intangible assets										66	94	80
Special items:
U.S. tax reform		196	0	0	0					196
Debt extinguishment costs	$ 65	0	209	0	0					209		65
Long-term care guaranty fund assessment		0	0	0	83					83
Transaction-related costs		$ 25	$ 6	$ (47)	$ 49	39	46	26	36	33	147	57
Risk corridor allowance						80	0	0	0		80
Charges associated with litigation matters						$ 0	$ 25	$ 0	$ 0		25
Adjusted income (loss) from operations										2,668	2,104	2,256
Bargain purchase gain												23
Operating Segments [Member] | Global Health Benefits Segment [Member]
Segment Reporting Information [Line Items]
Premiums										24,538	23,295	22,696
Fees and other revenues										4,722	4,623	4,357
Net investment income										378	315	340
Mail order pharmacy revenues										2,979	2,966	2,536
Total operating revenues										32,617	31,199	29,929
Net realized investment gains (losses)										136	119	43
Total revenues										32,753	31,318	29,972
Depreciation and amortization										477	526	526
Total benefits and expenses										29,440	28,467	27,028
Income (loss) before income taxes										3,313	2,851	2,944
Income taxes (benefits) and net (income) loss attributable to noncontrolling interests										1,031	1,100	1,150
Shareholders' Net Income										2,282	1,751	1,794
After-tax adjustments to reconcile to adjusted income from operations:
Realized investment (gains) losses										(88)	(78)	(30)
Amortization of other acquired intangible assets										48	74	84
Special items:
U.S. tax reform										(137)
Debt extinguishment costs										0		0
Long-term care guaranty fund assessment										68
Transaction-related costs										0	0	0
Risk corridor allowance											80
Charges associated with litigation matters											25
Adjusted income (loss) from operations										2,173	1,852	1,848
Operating Segments [Member] | Global Supplemental Benefits Segment [Member]
Segment Reporting Information [Line Items]
Premiums										3,684	3,226	3,000
Fees and other revenues										66	49	46
Net investment income										122	110	103
Mail order pharmacy revenues										0	0	0
Total operating revenues										3,872	3,385	3,149
Net realized investment gains (losses)										32	(5)	0
Total revenues										3,904	3,380	3,149
Depreciation and amortization										54	54	31
Total benefits and expenses										3,407	3,052	2,849
Income (loss) before income taxes										497	328	300
Income taxes (benefits) and net (income) loss attributable to noncontrolling interests										195	60	33
Shareholders' Net Income										302	268	267
After-tax adjustments to reconcile to adjusted income from operations:
Realized investment (gains) losses										(24)	6	(1)
Amortization of other acquired intangible assets										18	20	(4)
Special items:
U.S. tax reform										73
Debt extinguishment costs										0		0
Long-term care guaranty fund assessment										0
Transaction-related costs										0	0	0
Risk corridor allowance											0
Charges associated with litigation matters											0
Adjusted income (loss) from operations										369	294	262
Bargain purchase gain												23
Operating Segments [Member] | Group Disability And Life Segment [Member]
Segment Reporting Information [Line Items]
Premiums										3,985	4,002	3,843
Fees and other revenues										106	98	91
Net investment income										350	343	337
Mail order pharmacy revenues										0	0	0
Total operating revenues										4,441	4,443	4,271
Net realized investment gains (losses)										74	59	5
Total revenues										4,515	4,502	4,276
Depreciation and amortization										30	28	26
Total benefits and expenses										4,044	4,273	3,796
Income (loss) before income taxes										471	229	480
Income taxes (benefits) and net (income) loss attributable to noncontrolling interests										113	65	152
Shareholders' Net Income										358	164	328
After-tax adjustments to reconcile to adjusted income from operations:
Realized investment (gains) losses										(49)	(39)	(4)
Amortization of other acquired intangible assets										0	0	0
Special items:
U.S. tax reform										(39)
Debt extinguishment costs										0		0
Long-term care guaranty fund assessment										15
Transaction-related costs										0	0	0
Risk corridor allowance											0
Charges associated with litigation matters											0
Adjusted income (loss) from operations										285	125	324
Operating Segments [Member] | Other Operations Segment [Member]
Segment Reporting Information [Line Items]
Premiums										112	103	103
Fees and other revenues										10	11	13
Net investment income										346	358	369
Mail order pharmacy revenues										0	0	0
Total operating revenues										468	472	485
Net realized investment gains (losses)										(5)	(5)	9
Total revenues										463	467	494
Depreciation and amortization										1	1	1
Total benefits and expenses										316	369	374
Income (loss) before income taxes										147	98	120
Income taxes (benefits) and net (income) loss attributable to noncontrolling interests										222	30	40
Shareholders' Net Income										(75)	68	80
After-tax adjustments to reconcile to adjusted income from operations:
Realized investment (gains) losses										4	2	(5)
Amortization of other acquired intangible assets										0	0	0
Special items:
U.S. tax reform										138
Debt extinguishment costs										0		0
Long-term care guaranty fund assessment										0
Transaction-related costs										0	0	0
Risk corridor allowance											0
Charges associated with litigation matters											0
Adjusted income (loss) from operations										67	70	75
Corporate [Member]
Segment Reporting Information [Line Items]
Premiums										(12)	0	0
Fees and other revenues										(37)	(21)	(19)
Net investment income										30	21	4
Mail order pharmacy revenues										0	0	0
Total operating revenues										(19)	0	(15)
Net realized investment gains (losses)										0	1	0
Total revenues										(19)	1	(15)
Depreciation and amortization										4	1	1
Total benefits and expenses										803	528	502
Income (loss) before income taxes										(822)	(527)	(517)
Income taxes (benefits) and net (income) loss attributable to noncontrolling interests										(192)	(143)	(142)
Shareholders' Net Income										(630)	(384)	(375)
After-tax adjustments to reconcile to adjusted income from operations:
Realized investment (gains) losses										1	0	0
Amortization of other acquired intangible assets										0	0	0
Special items:
U.S. tax reform										161
Debt extinguishment costs										209		65
Long-term care guaranty fund assessment										0
Transaction-related costs										33	147	57
Risk corridor allowance											0
Charges associated with litigation matters											0
Adjusted income (loss) from operations										$ (226)	$ (237)	$ (253)