Segment Information - Summarized Segment Financial Information (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Segment Reporting Information [Line Items]
Revenues from external customers									$ 47,170	$ 40,580	$ 38,691
Net investment income									1,480	1,226	1,147
Total revenues	$ 14,300	$ 11,457	$ 11,480	$ 11,413	$ 10,632	$ 10,372	$ 10,374	$ 10,428	48,650	41,806	39,838
Revenue from transitioning clients									(459)
Net realized investment results from equity method subsidiaries									43
Special items reported in transaction-related costs									(123)
Adjusted revenues									(48,111)	41,806	39,838
Depreciation and amortization									695	566	610
Income (loss) before income taxes	$ 228	$ 1,033	$ 1,102	$ 1,218	$ 758	824	$ 1,134	$ 890	3,581	3,606	2,979
Pre-tax adjustments to reconcile adjusted income from operations [Abstract]
Adjustment for transitioning clients									(62)
(Income) loss attributable to non-controlling interests									(14)	2	20
Net realized investments (gains) losses									124	(237)	(169)
Amortization of acquired intangible assets									235	115	151
Special items [Abstract]
Transaction-related costs									852	126	166
Charges associated with litigation matters									25		40
U.S. Tax reform									2	(56)
Debt extinguishment costs						$ 321			0	321	0
Long-term care guaranty fund assessment										129
Risk corridor allowance											124
Pre-tax adjusted income (loss) from operations									4,743	4,006	3,311
Integrated Medical [Member]
Segment Reporting Information [Line Items]
Revenues from external customers									31,759	28,193	26,695
Health Services [Member]
Segment Reporting Information [Line Items]
Revenues from external customers									5,902	3,250	3,169
International Markets [Member]
Segment Reporting Information [Line Items]
Revenues from external customers									5,174	4,774	4,424
Group Disability and Other [Member]
Segment Reporting Information [Line Items]
Revenues from external customers									4,335	4,363	4,403
Operating Segments [Member] | Integrated Medical [Member]
Segment Reporting Information [Line Items]
Net investment income									459	366	305
Total revenues									32,791	29,035	27,395
Revenue from transitioning clients									0
Net realized investment results from equity method subsidiaries									0
Special items reported in transaction-related costs									0
Adjusted revenues									32,791	29,035	27,395
Depreciation and amortization									466	470	519
Income (loss) before income taxes									3,342	2,859	2,417
Pre-tax adjustments to reconcile adjusted income from operations [Abstract]
Adjustment for transitioning clients									0
(Income) loss attributable to non-controlling interests									0	1	2
Net realized investments (gains) losses									36	(137)	(116)
Amortization of acquired intangible assets									99	93	125
Special items [Abstract]
Transaction-related costs									0	0	0
Charges associated with litigation matters									25		40
U.S. Tax reform									0	0
Debt extinguishment costs										0
Long-term care guaranty fund assessment										106
Risk corridor allowance											124
Pre-tax adjusted income (loss) from operations									3,502	2,922	2,592
Operating Segments [Member] | Health Services [Member]
Segment Reporting Information [Line Items]
Net investment income									9	3	3
Total revenues									7,065	4,241	4,066
Revenue from transitioning clients									(459)
Net realized investment results from equity method subsidiaries									0
Special items reported in transaction-related costs									0
Adjusted revenues									6,606	4,241	4,066
Depreciation and amortization									120	0	0
Income (loss) before income taxes									329	288	268
Pre-tax adjustments to reconcile adjusted income from operations [Abstract]
Adjustment for transitioning clients									(62)
(Income) loss attributable to non-controlling interests									0	0	0
Net realized investments (gains) losses									0	0	0
Amortization of acquired intangible assets									113	0	0
Special items [Abstract]
Transaction-related costs									0	0	0
Charges associated with litigation matters									0		0
U.S. Tax reform									0	0
Debt extinguishment costs										0
Long-term care guaranty fund assessment										0
Risk corridor allowance											0
Pre-tax adjusted income (loss) from operations									380	288	268
Operating Segments [Member] | International Markets [Member]
Segment Reporting Information [Line Items]
Net investment income									149	127	113
Total revenues									5,323	4,901	4,537
Revenue from transitioning clients									0
Net realized investment results from equity method subsidiaries									43
Special items reported in transaction-related costs									0
Adjusted revenues									5,366	4,901	4,537
Depreciation and amortization									55	61	61
Income (loss) before income taxes									670	667	497
Pre-tax adjustments to reconcile adjusted income from operations [Abstract]
Adjustment for transitioning clients									0
(Income) loss attributable to non-controlling interests									(14)	1	18
Net realized investments (gains) losses									61	(31)	2
Amortization of acquired intangible assets									18	17	21
Special items [Abstract]
Transaction-related costs									0	0	0
Charges associated with litigation matters									0		0
U.S. Tax reform									0	0
Debt extinguishment costs										0
Long-term care guaranty fund assessment										0
Risk corridor allowance											0
Pre-tax adjusted income (loss) from operations									735	654	538
Operating Segments [Member] | Group Disability and Other [Member]
Segment Reporting Information [Line Items]
Net investment income									712	700	705
Total revenues									5,061	5,075	5,108
Revenue from transitioning clients									0
Net realized investment results from equity method subsidiaries									0
Special items reported in transaction-related costs									0
Adjusted revenues									5,061	5,075	5,108
Depreciation and amortization									53	31	29
Income (loss) before income taxes									497	614	324
Pre-tax adjustments to reconcile adjusted income from operations [Abstract]
Adjustment for transitioning clients									0
(Income) loss attributable to non-controlling interests									0	0	0
Net realized investments (gains) losses									25	(69)	(54)
Amortization of acquired intangible assets									5	5	5
Special items [Abstract]
Transaction-related costs									0	0	0
Charges associated with litigation matters									0		0
U.S. Tax reform									2	(56)
Debt extinguishment costs										0
Long-term care guaranty fund assessment										23
Risk corridor allowance											0
Pre-tax adjusted income (loss) from operations									529	517	275
Corporate and eliminations [Member]
Segment Reporting Information [Line Items]
Total revenues									(1,590)	(1,446)	(1,268)
Revenue from transitioning clients									0
Net realized investment results from equity method subsidiaries									0
Special items reported in transaction-related costs									(123)
Adjusted revenues									(1,713)	(1,446)	(1,268)
Depreciation and amortization									1	4	1
Income (loss) before income taxes									(1,257)	(822)	(527)
Pre-tax adjustments to reconcile adjusted income from operations [Abstract]
Adjustment for transitioning clients									0
(Income) loss attributable to non-controlling interests									0	0	0
Net realized investments (gains) losses									2	0	(1)
Amortization of acquired intangible assets									0	0	0
Special items [Abstract]
Transaction-related costs									852	126	166
Charges associated with litigation matters									0		0
U.S. Tax reform									0	0
Debt extinguishment costs										321
Long-term care guaranty fund assessment										0
Risk corridor allowance											0
Pre-tax adjusted income (loss) from operations									(403)	(375)	(362)
Corporate Non-segment [Member]
Segment Reporting Information [Line Items]
Revenues from external customers									0	0	0
Net investment income									151	30	21
Intersegment eliminations [Member]
Segment Reporting Information [Line Items]
Revenues from external customers									(1,741)	(1,476)	(1,289)
Intersegment eliminations [Member] | Integrated Medical [Member]
Segment Reporting Information [Line Items]
Revenues from external customers									(573)	(476)	(395)
Intersegment eliminations [Member] | Health Services [Member]
Segment Reporting Information [Line Items]
Revenues from external customers									(1,154)	(988)	(894)
Intersegment eliminations [Member] | International Markets [Member]
Segment Reporting Information [Line Items]
Revenues from external customers									0	0	0
Intersegment eliminations [Member] | Group Disability and Other [Member]
Segment Reporting Information [Line Items]
Revenues from external customers									$ (14)	$ (12)	$ 0