Summary of Significant Accounting Policies - Revenue Recognition (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Operating results by business segment
Net revenue	$ 1,184,960	$ 1,170,117	$ 1,100,075	$ 1,122,379	$ 1,133,775	$ 1,158,543	$ 1,154,289	$ 1,119,006	$ 4,577,531	$ 4,565,613	$ 4,957,522
Transferred at a point in time
Operating results by business segment
Net revenue									2,327,510	2,218,678	2,606,946
Transferred over time
Operating results by business segment
Net revenue									2,250,021	2,346,935	2,350,576
Government
Operating results by business segment
Net revenue									1,756,819	1,748,215	1,846,121
Non-government
Operating results by business segment
Net revenue									2,820,712	2,817,398	3,111,401
Behavioral & Specialty Health, risk-based, non-EAP
Operating results by business segment
Net revenue									1,399,144	1,504,182	1,511,269
Behavioral & Specialty Health, EAP risk-based
Operating results by business segment
Net revenue									315,306	339,377	349,751
Behavioral & Specialty Health, ASO
Operating results by business segment
Net revenue									291,608	278,285	283,259
PBM, including dispensing
Operating results by business segment
Net revenue									2,083,766	1,931,074	2,164,680
Medicare Part D
Operating results by business segment
Net revenue									243,744	287,604	442,266
PBA
Operating results by business segment
Net revenue									136,155	137,885	132,112
Formulary management
Operating results by business segment
Net revenue									104,891	84,567	70,900
Other service lines
Operating results by business segment
Net revenue									2,917	2,639	3,285
Healthcare
Operating results by business segment
Net revenue									1,959,869	2,082,088	2,110,756
Healthcare | Transferred over time
Operating results by business segment
Net revenue									1,959,869	2,082,088	2,110,756
Healthcare | Government
Operating results by business segment
Net revenue									935,138	916,542	899,515
Healthcare | Non-government
Operating results by business segment
Net revenue									1,024,731	1,165,546	1,211,241
Healthcare | Behavioral & Specialty Health, risk-based, non-EAP
Operating results by business segment
Net revenue									1,399,532	1,504,472	1,511,532
Healthcare | Behavioral & Specialty Health, EAP risk-based
Operating results by business segment
Net revenue									315,306	339,377	349,751
Healthcare | Behavioral & Specialty Health, ASO
Operating results by business segment
Net revenue									245,031	238,239	249,473
Pharmacy Management
Operating results by business segment
Net revenue									2,638,301	2,502,268	2,865,844
Pharmacy Management | Transferred at a point in time
Operating results by business segment
Net revenue									2,347,446	2,236,829	2,625,417
Pharmacy Management | Transferred over time
Operating results by business segment
Net revenue									290,855	265,439	240,427
Pharmacy Management | Government
Operating results by business segment
Net revenue									821,681	831,673	946,606
Pharmacy Management | Non-government
Operating results by business segment
Net revenue									1,816,620	1,670,595	1,919,238
Pharmacy Management | Behavioral & Specialty Health, ASO
Operating results by business segment
Net revenue									46,892	40,348	34,130
Pharmacy Management | PBM, including dispensing
Operating results by business segment
Net revenue									2,103,702	1,949,225	2,183,151
Pharmacy Management | Medicare Part D
Operating results by business segment
Net revenue									243,744	287,604	442,266
Pharmacy Management | PBA
Operating results by business segment
Net revenue									136,155	137,885	132,112
Pharmacy Management | Formulary management
Operating results by business segment
Net revenue									104,891	84,567	70,900
Pharmacy Management | Other service lines
Operating results by business segment
Net revenue									2,917	2,639	3,285
Elimination
Operating results by business segment
Net revenue									(20,639)	(18,743)	(19,078)
Elimination | Transferred at a point in time
Operating results by business segment
Net revenue									(19,936)	(18,151)	(18,471)
Elimination | Transferred over time
Operating results by business segment
Net revenue									(703)	(592)	(607)
Elimination | Non-government
Operating results by business segment
Net revenue									(20,639)	(18,743)	(19,078)
Elimination | Behavioral & Specialty Health, risk-based, non-EAP
Operating results by business segment
Net revenue									(388)	(290)	(263)
Elimination | Behavioral & Specialty Health, ASO
Operating results by business segment
Net revenue									(315)	(302)	(344)
Elimination | PBM, including dispensing
Operating results by business segment
Net revenue									$ (19,936)	$ (18,151)	$ (18,471)