Segment Information - Summary of Special Items (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2020	Dec. 31, 2019	Mar. 31, 2019
After-tax
Debt extinguishment costs (Debt extinguishment costs)	$ 140		$ 0
Integration and transaction-related costs (Selling, general and administrative expenses)	74		108
Charge for organizational efficiency plan (Selling, general and administrative expenses)	24	$ 162	0
Charges associated with litigation matters (Selling, general and administrative expenses)	19		0
Contractual adjustment for a former client (Pharmacy revenues)	(66)		0
Total impact from special items	191		108
Before-tax
Debt extinguishment costs (Debt extinguishment costs)	185		0
Integration and transaction-related costs (Selling, general and administrative expenses)	97		136
Charge for organizational efficiency plan (Selling, general and administrative expenses)	31	$ 207	0
Charges associated with litigation matters (Selling, general and administrative expenses)	25		0
Contractual adjustment for a former client (Pharmacy revenues)	(87)		0
Total impact from special items	$ 251		$ 136