Segment Information - Summarized Segment Financial Information (Details) - USD ($) $ in Millions	3 Months Ended				9 Months Ended
	Sep. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Segment Reporting Information [Line Items]
Revenues from customers	$ 40,658			$ 38,207	$ 117,816	$ 114,286
Net investment income	297			349	873	1,035
Total revenues	40,955			38,556	118,689	115,321
Revenue contribution from transitioning clients				(2,718)		(11,657)
Net realized investment results from certain equity method investments	(37)			(5)	(87)	(27)
Special item related to contractual adjustment for a former client	(117)				(204)
Adjusted revenues	40,801			35,833	118,398	103,637
Income before income taxes	1,804			1,763	5,490	5,309
Pre-tax adjustments to reconcile to adjusted income from operations
Adjustment for transitioning clients				(274)		(1,589)
(Income) attributable to noncontrolling interests	(10)			(4)	(27)	(13)
Net realized investments (gains) losses	(69)			(56)	(69)	(111)
Amortization of acquired intangible assets	493			734	1,487	2,214
Special items
Integration and transaction-related costs	112			114	339	405
Debt extinguishment costs					199	0
Charge for organizational efficiency plan		$ 31	$ 207		31	0
(Benefits) charges associated with litigation matters	0			(30)	25	51
Risk corridors recovery (Selling, general and administrative expenses)	(101)			0	(101)	0
Contractual adjustment for a former client	(117)			0	(204)	0
Pre-tax adjusted income (loss) from operations	2,112			2,247	7,170	6,266
Evernorth
Segment Reporting Information [Line Items]
Revenues from customers	29,016			27,084	82,986	80,618
U.S. Medical
Segment Reporting Information [Line Items]
Revenues from customers	9,047			8,648	26,999	26,226
International Markets
Segment Reporting Information [Line Items]
Revenues from customers	1,439			1,368	4,325	4,095
Group Disability and Other
Segment Reporting Information [Line Items]
Revenues from customers	1,156			1,107	3,506	3,347
Operating Segments | Evernorth
Segment Reporting Information [Line Items]
Net investment income	2			15	30	47
Total revenues	29,944			27,598	85,801	82,534
Revenue contribution from transitioning clients				(2,718)		(11,657)
Net realized investment results from certain equity method investments	0			0	0	0
Special item related to contractual adjustment for a former client	(117)				(204)
Adjusted revenues	29,827			24,880	85,597	70,877
Income before income taxes	1,086			965	2,551	3,015
Pre-tax adjustments to reconcile to adjusted income from operations
Adjustment for transitioning clients				(274)		(1,589)
(Income) attributable to noncontrolling interests	(5)			0	(12)	(1)
Net realized investments (gains) losses	0			0	0	0
Amortization of acquired intangible assets	479			708	1,439	2,130
Special items
Integration and transaction-related costs	0			0	0	0
Debt extinguishment costs					0
Charge for organizational efficiency plan					0
(Benefits) charges associated with litigation matters				0	0	0
Risk corridors recovery (Selling, general and administrative expenses)	0				0
Contractual adjustment for a former client	(117)				(204)
Pre-tax adjusted income (loss) from operations	1,443			1,399	3,774	3,555
Operating Segments | U.S. Medical
Segment Reporting Information [Line Items]
Net investment income	104			124	279	358
Total revenues	9,629			9,148	28,726	27,311
Revenue contribution from transitioning clients				0		0
Net realized investment results from certain equity method investments	0			0	0	0
Special item related to contractual adjustment for a former client	0				0
Adjusted revenues	9,629			9,148	28,726	27,311
Income before income taxes	846			1,011	3,529	3,157
Pre-tax adjustments to reconcile to adjusted income from operations
Adjustment for transitioning clients				0		0
(Income) attributable to noncontrolling interests	0			0	0	0
Net realized investments (gains) losses	6			(43)	28	(65)
Amortization of acquired intangible assets	6			15	23	51
Special items
Integration and transaction-related costs	0			0	0	0
Debt extinguishment costs					0
Charge for organizational efficiency plan					0
(Benefits) charges associated with litigation matters				(30)	0	(30)
Risk corridors recovery (Selling, general and administrative expenses)	(101)				(101)
Contractual adjustment for a former client	0				0
Pre-tax adjusted income (loss) from operations	757			953	3,479	3,113
Operating Segments | International Markets
Segment Reporting Information [Line Items]
Net investment income	38			39	104	117
Total revenues	1,477			1,407	4,429	4,212
Revenue contribution from transitioning clients				0		0
Net realized investment results from certain equity method investments	(37)			(5)	(87)	(27)
Special item related to contractual adjustment for a former client	0				0
Adjusted revenues	1,440			1,402	4,342	4,185
Income before income taxes	253			184	877	601
Pre-tax adjustments to reconcile to adjusted income from operations
Adjustment for transitioning clients				0		0
(Income) attributable to noncontrolling interests	(5)			(4)	(15)	(12)
Net realized investments (gains) losses	(48)			5	(75)	(10)
Amortization of acquired intangible assets	8			9	22	28
Special items
Integration and transaction-related costs	0			0	0	0
Debt extinguishment costs					0
Charge for organizational efficiency plan					0
(Benefits) charges associated with litigation matters				0	0	0
Risk corridors recovery (Selling, general and administrative expenses)	0				0
Contractual adjustment for a former client	0				0
Pre-tax adjusted income (loss) from operations	208			194	809	607
Operating Segments | Group Disability and Other
Segment Reporting Information [Line Items]
Net investment income	152			170	458	521
Total revenues	1,314			1,284	3,981	3,889
Revenue contribution from transitioning clients				0		0
Net realized investment results from certain equity method investments	0			0	0	0
Special item related to contractual adjustment for a former client	0				0
Adjusted revenues	1,314			1,284	3,981	3,889
Income before income taxes	97			159	298	407
Pre-tax adjustments to reconcile to adjusted income from operations
Adjustment for transitioning clients				0		0
(Income) attributable to noncontrolling interests	0			0	0	0
Net realized investments (gains) losses	(27)			(18)	(22)	(36)
Amortization of acquired intangible assets	0			2	3	5
Special items
Integration and transaction-related costs	0			0	0	0
Debt extinguishment costs					0
Charge for organizational efficiency plan					0
(Benefits) charges associated with litigation matters				0	0	0
Risk corridors recovery (Selling, general and administrative expenses)	0				0
Contractual adjustment for a former client	0				0
Pre-tax adjusted income (loss) from operations	70			143	279	376
Corporate and Eliminations
Segment Reporting Information [Line Items]
Net investment income	1			1	2	(8)
Total revenues	(1,409)			(881)	(4,248)	(2,625)
Revenue contribution from transitioning clients				0		0
Net realized investment results from certain equity method investments	0			0	0	0
Special item related to contractual adjustment for a former client	0				0
Adjusted revenues	(1,409)			(881)	(4,248)	(2,625)
Income before income taxes	(478)			(556)	(1,765)	(1,871)
Special items
Pre-tax adjusted income (loss) from operations	(366)			(442)	(1,171)	(1,385)
Corporate
Segment Reporting Information [Line Items]
Revenues from customers	0			0	0	0
Pre-tax adjustments to reconcile to adjusted income from operations
Adjustment for transitioning clients				0		0
(Income) attributable to noncontrolling interests	0			0	0	0
Net realized investments (gains) losses	0			0	0	0
Amortization of acquired intangible assets	0			0	0	0
Special items
Integration and transaction-related costs	112			114	339	405
Debt extinguishment costs					199
Charge for organizational efficiency plan					31
(Benefits) charges associated with litigation matters				0	25	81
Risk corridors recovery (Selling, general and administrative expenses)	0				0
Contractual adjustment for a former client	0				0
Intersegment Eliminations
Segment Reporting Information [Line Items]
Revenues from customers	(1,410)			(882)	(4,250)	(2,617)
Intersegment Eliminations | Evernorth
Segment Reporting Information [Line Items]
Revenues from customers	(926)			(499)	(2,785)	(1,869)
Intersegment Eliminations | U.S. Medical
Segment Reporting Information [Line Items]
Revenues from customers	(478)			(376)	(1,448)	(727)
Intersegment Eliminations | International Markets
Segment Reporting Information [Line Items]
Revenues from customers	0			0	0	0
Intersegment Eliminations | Group Disability and Other
Segment Reporting Information [Line Items]
Revenues from customers	$ (6)			$ (7)	$ (17)	$ (21)