Segment Information - Summarized Segment Financial Information (Details) - USD ($) $ in Millions	3 Months Ended
Mar. 31, 2020	Dec. 31, 2019	Mar. 31, 2019
Segment Reporting Information [Line Items]
Revenues from external customers	$ 38,116		$ 37,600
Net investment income	353		346
Total revenues	38,469		37,946
Revenue contribution from transitioning clients			(4,489)
Net realized investment results from certain equity method investments	10		(28)
Special items	(87)
Adjusted revenues	38,392		33,429
Income before income taxes	1,397		1,788
Pre-tax adjustments to reconcile to adjusted income from operations
Adjustment for transitioning clients			(660)
(Income) attributable to noncontrolling interests	(9)		(5)
Net realized investments (gains) losses	98		(38)
Amortization of acquired intangible assets	498		743
Special items
Debt extinguishment costs	185		0
Integration and transaction-related costs	97		136
Charge for organizational efficiency plan	31	$ 207	0
Charges associated with litigation matters	25		0
Contractual adjustment for a former client	(87)		0
Pre-tax adjusted income (loss) from operations	2,235		1,964
Operating Segments \| Health Services
Segment Reporting Information [Line Items]
Revenues from external customers	26,256		26,197
Net investment income	25		15
Total revenues	27,255		26,949
Revenue contribution from transitioning clients			(4,489)
Net realized investment results from certain equity method investments	0		0
Special items	(87)
Adjusted revenues	27,168		22,460
Income before income taxes	694		942
Pre-tax adjustments to reconcile to adjusted income from operations
Adjustment for transitioning clients			(660)
(Income) attributable to noncontrolling interests	(4)		(1)
Net realized investments (gains) losses	0		0
Amortization of acquired intangible assets	479		713
Special items
Debt extinguishment costs	0
Integration and transaction-related costs	0		0
Charge for organizational efficiency plan	0
Charges associated with litigation matters	0
Contractual adjustment for a former client	(87)
Pre-tax adjusted income (loss) from operations	1,082		994
Operating Segments \| Integrated Medical
Segment Reporting Information [Line Items]
Revenues from external customers	9,268		8,910
Net investment income	126		123
Total revenues	9,860		9,195
Revenue contribution from transitioning clients			0
Net realized investment results from certain equity method investments	0		0
Special items	0
Adjusted revenues	9,860		9,195
Income before income taxes	1,140		1,157
Pre-tax adjustments to reconcile to adjusted income from operations
Adjustment for transitioning clients			0
(Income) attributable to noncontrolling interests	0		0
Net realized investments (gains) losses	48		(5)
Amortization of acquired intangible assets	11		18
Special items
Debt extinguishment costs	0
Integration and transaction-related costs	0		0
Charge for organizational efficiency plan	0
Charges associated with litigation matters	0
Contractual adjustment for a former client	0
Pre-tax adjusted income (loss) from operations	1,199		1,170
Operating Segments \| International Markets
Segment Reporting Information [Line Items]
Revenues from external customers	1,420		1,384
Net investment income	40		38
Total revenues	1,460		1,422
Revenue contribution from transitioning clients			0
Net realized investment results from certain equity method investments	10		(28)
Special items	0
Adjusted revenues	1,470		1,394
Income before income taxes	234		222
Pre-tax adjustments to reconcile to adjusted income from operations
Adjustment for transitioning clients			0
(Income) attributable to noncontrolling interests	(5)		(4)
Net realized investments (gains) losses	46		(23)
Amortization of acquired intangible assets	7		11
Special items
Debt extinguishment costs	0
Integration and transaction-related costs	0		0
Charge for organizational efficiency plan	0
Charges associated with litigation matters	0
Contractual adjustment for a former client	0
Pre-tax adjusted income (loss) from operations	282		206
Operating Segments \| Group Disability and Other
Segment Reporting Information [Line Items]
Revenues from external customers	1,172		1,109
Net investment income	162		180
Total revenues	1,339		1,296
Revenue contribution from transitioning clients			0
Net realized investment results from certain equity method investments	0		0
Special items	0
Adjusted revenues	1,339		1,296
Income before income taxes	72		93
Pre-tax adjustments to reconcile to adjusted income from operations
Adjustment for transitioning clients			0
(Income) attributable to noncontrolling interests	0		0
Net realized investments (gains) losses	4		(10)
Amortization of acquired intangible assets	1		1
Special items
Debt extinguishment costs	0
Integration and transaction-related costs	0		0
Charge for organizational efficiency plan	0
Charges associated with litigation matters	0
Contractual adjustment for a former client	0
Pre-tax adjusted income (loss) from operations	77		84
Corporate and Eliminations
Segment Reporting Information [Line Items]
Net investment income	0		(10)
Total revenues	(1,445)		(916)
Revenue contribution from transitioning clients			0
Net realized investment results from certain equity method investments	0		0
Special items	0
Adjusted revenues	(1,445)		(916)
Income before income taxes	(743)		(626)
Special items
Pre-tax adjusted income (loss) from operations	(405)		(490)
Corporate
Segment Reporting Information [Line Items]
Revenues from external customers	0		0
Pre-tax adjustments to reconcile to adjusted income from operations
Adjustment for transitioning clients			0
(Income) attributable to noncontrolling interests	0		0
Net realized investments (gains) losses	0		0
Amortization of acquired intangible assets	0		0
Special items
Debt extinguishment costs	185
Integration and transaction-related costs	97		136
Charge for organizational efficiency plan	31
Charges associated with litigation matters	25
Contractual adjustment for a former client	0
Intersegment Eliminations
Segment Reporting Information [Line Items]
Revenues from external customers	(1,445)		(906)
Intersegment Eliminations \| Health Services
Segment Reporting Information [Line Items]
Revenues from external customers	974		737
Intersegment Eliminations \| Integrated Medical
Segment Reporting Information [Line Items]
Revenues from external customers	466		162
Intersegment Eliminations \| International Markets
Segment Reporting Information [Line Items]
Revenues from external customers	0		0
Intersegment Eliminations \| Group Disability and Other
Segment Reporting Information [Line Items]
Revenues from external customers	$ 5		$ 7

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