General - Revenue Recognition (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Operating results by business segment
Net revenue	$ 1,170,117	$ 1,158,543	$ 3,392,571	$ 3,431,838
Transferred at a point in time
Operating results by business segment
Net revenue	601,429	567,314	1,722,004	1,665,447
Transferred over time
Operating results by business segment
Net revenue	568,688	591,229	1,670,567	1,766,391
Government
Operating results by business segment
Net revenue	450,885	493,081	1,313,326	1,319,103
Non-government
Operating results by business segment
Net revenue	719,232	665,462	2,079,245	2,112,735
Behavioral & Specialty Health, risk-based, non-EAP
Operating results by business segment
Net revenue	352,347	383,692	1,046,209	1,146,373
Behavioral & Specialty Health, EAP risk-based
Operating results by business segment
Net revenue	74,703	80,076	232,060	256,989
Behavioral & Specialty Health, ASO
Operating results by business segment
Net revenue	73,887	66,968	216,050	198,463
PBM, including dispensing
Operating results by business segment
Net revenue	535,498	490,987	1,543,696	1,455,937
Medicare Part D
Operating results by business segment
Net revenue	65,931	76,327	178,308	209,510
PBA
Operating results by business segment
Net revenue	35,068	35,768	96,008	103,220
Formulary management
Operating results by business segment
Net revenue	31,327	23,976	78,550	59,585
Other service lines
Operating results by business segment
Net revenue	1,356	749	1,690	1,761
Healthcare
Operating results by business segment
Net revenue	489,451	521,379	1,459,378	1,574,314
Healthcare | Transferred over time
Operating results by business segment
Net revenue	489,451	521,379	1,459,378	1,574,314
Healthcare | Government
Operating results by business segment
Net revenue	240,845	277,884	690,340	694,534
Healthcare | Non-government
Operating results by business segment
Net revenue	248,606	243,495	769,038	879,780
Healthcare | Behavioral & Specialty Health, risk-based, non-EAP
Operating results by business segment
Net revenue	352,442	383,768	1,046,486	1,146,592
Healthcare | Behavioral & Specialty Health, EAP risk-based
Operating results by business segment
Net revenue	74,703	80,076	232,060	256,989
Healthcare | Behavioral & Specialty Health, ASO
Operating results by business segment
Net revenue	62,306	57,535	180,832	170,733
Pharmacy Management
Operating results by business segment
Net revenue	685,928	642,054	1,948,203	1,871,172
Pharmacy Management | Transferred at a point in time
Operating results by business segment
Net revenue	606,546	572,086	1,736,519	1,678,661
Pharmacy Management | Transferred over time
Operating results by business segment
Net revenue	79,382	69,968	211,684	192,511
Pharmacy Management | Government
Operating results by business segment
Net revenue	210,040	215,197	622,986	624,569
Pharmacy Management | Non-government
Operating results by business segment
Net revenue	475,888	426,857	1,325,217	1,246,603
Pharmacy Management | Behavioral & Specialty Health, ASO
Operating results by business segment
Net revenue	11,631	9,475	35,436	27,945
Pharmacy Management | PBM, including dispensing
Operating results by business segment
Net revenue	540,615	495,759	1,558,211	1,469,151
Pharmacy Management | Medicare Part D
Operating results by business segment
Net revenue	65,931	76,327	178,308	209,510
Pharmacy Management | PBA
Operating results by business segment
Net revenue	35,068	35,768	96,008	103,220
Pharmacy Management | Formulary management
Operating results by business segment
Net revenue	31,327	23,976	78,550	59,585
Pharmacy Management | Other service lines
Operating results by business segment
Net revenue	1,356	749	1,690	1,761
Elimination
Operating results by business segment
Net revenue	(5,262)	(4,890)	(15,010)	(13,648)
Elimination | Transferred at a point in time
Operating results by business segment
Net revenue	(5,117)	(4,772)	(14,515)	(13,214)
Elimination | Transferred over time
Operating results by business segment
Net revenue	(145)	(118)	(495)	(434)
Elimination | Non-government
Operating results by business segment
Net revenue	(5,262)	(4,890)	(15,010)	(13,648)
Elimination | Behavioral & Specialty Health, risk-based, non-EAP
Operating results by business segment
Net revenue	(95)	(76)	(277)	(219)
Elimination | Behavioral & Specialty Health, ASO
Operating results by business segment
Net revenue	(50)	(42)	(218)	(215)
Elimination | PBM, including dispensing
Operating results by business segment
Net revenue	$ (5,117)	$ (4,772)	$ (14,515)	$ (13,214)