Segment Information - Summary of Special Items (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
After-tax
Integration and transaction-related costs (Selling, general and administrative expenses)	$ 83			$ 88	$ 256	$ 311
Debt extinguishment costs (Debt extinguishment costs)					151	0
Charge for organizational efficiency plan (Selling, general and administrative expenses)		$ 24	$ 162		24	0
(Benefits) charges associated with litigation matters (Selling, general and administrative expenses)	0			(23)	19	41
Risk corridors recovery (Selling, general and administrative expenses)	(76)			0	(76)	0
Contractual adjustment for a former client (Pharmacy revenues)	(89)			0	(155)	0
Total impact from special items	(82)			65	219	352
Before-tax
Integration and transaction-related costs (Selling, general and administrative expenses)	112			114	339	405
Debt extinguishment costs (Debt extinguishment costs)					199	0
Charge for organizational efficiency plan (Selling, general and administrative expenses)		$ 31	$ 207		31	0
(Benefits) charges associated with litigation matters (Selling, general and administrative expenses)	0			(30)	25	51
Risk corridors recovery (Selling, general and administrative expenses)	(101)			0	(101)	0
Contractual adjustment for a former client (Pharmacy revenues)	(117)			0	(204)	0
Total impact from special items	$ (106)			$ 84	$ 289	$ 456