General – Revenue Recognition (Details) - USD ($) $ in Thousands	3 Months Ended	6 Months Ended
	Jun. 30, 2018	Jun. 30, 2018
Operating results by business segment
Net revenue	$ 1,810,923	$ 3,616,000
Transferred at a point in time
Operating results by business segment
Net revenue	595,583	1,180,897
Transferred over time
Operating results by business segment
Net revenue	1,215,340	2,435,103
PBM, including dispensing
Operating results by business segment
Net revenue	488,920	975,828
Medicare Part D
Operating results by business segment
Net revenue	106,663	205,069
PBA
Operating results by business segment
Net revenue	33,088	66,634
Formulary management
Operating results by business segment
Net revenue	18,348	38,725
Other service lines
Operating results by business segment
Net revenue	638	1,722
Commercial | Risk-based, non-EAP
Operating results by business segment
Net revenue	238,158	472,549
Commercial | EAP risk-based
Operating results by business segment
Net revenue	12,335	24,756
Commercial | ASO
Operating results by business segment
Net revenue	59,603	119,327
Government
Operating results by business segment
Net revenue	1,083,118	2,169,387
Government | Risk-based, non-EAP
Operating results by business segment
Net revenue	750,162	1,502,652
Government | EAP risk-based
Operating results by business segment
Net revenue	78,940	161,177
Government | ASO
Operating results by business segment
Net revenue	24,068	47,561
Non-government
Operating results by business segment
Net revenue	727,805	1,446,613
Healthcare
Operating results by business segment
Net revenue	1,154,888	2,312,489
Healthcare | Transferred over time
Operating results by business segment
Net revenue	1,154,888	2,312,489
Healthcare | Commercial | Risk-based, non-EAP
Operating results by business segment
Net revenue	238,272	472,663
Healthcare | Commercial | EAP risk-based
Operating results by business segment
Net revenue	12,335	24,756
Healthcare | Commercial | ASO
Operating results by business segment
Net revenue	51,111	103,680
Healthcare | Government
Operating results by business segment
Net revenue	853,170	1,711,390
Healthcare | Government | Risk-based, non-EAP
Operating results by business segment
Net revenue	750,162	1,502,652
Healthcare | Government | EAP risk-based
Operating results by business segment
Net revenue	78,940	161,177
Healthcare | Government | ASO
Operating results by business segment
Net revenue	24,068	47,561
Healthcare | Non-government
Operating results by business segment
Net revenue	301,718	601,099
Pharmacy Management
Operating results by business segment
Net revenue	703,397	1,397,902
Pharmacy Management | Transferred at a point in time
Operating results by business segment
Net revenue	642,794	1,274,992
Pharmacy Management | Transferred over time
Operating results by business segment
Net revenue	60,603	122,910
Pharmacy Management | PBM, including dispensing
Operating results by business segment
Net revenue	536,131	1,069,923
Pharmacy Management | Medicare Part D
Operating results by business segment
Net revenue	106,663	205,069
Pharmacy Management | PBA
Operating results by business segment
Net revenue	33,088	66,634
Pharmacy Management | Formulary management
Operating results by business segment
Net revenue	18,348	38,725
Pharmacy Management | Other service lines
Operating results by business segment
Net revenue	638	1,722
Pharmacy Management | Commercial | ASO
Operating results by business segment
Net revenue	8,529	15,829
Pharmacy Management | Government
Operating results by business segment
Net revenue	229,948	457,997
Pharmacy Management | Non-government
Operating results by business segment
Net revenue	473,449	939,905
Elimination
Operating results by business segment
Net revenue	(47,362)	(94,391)
Elimination | Transferred at a point in time
Operating results by business segment
Net revenue	(47,211)	(94,095)
Elimination | Transferred over time
Operating results by business segment
Net revenue	(151)	(296)
Elimination | PBM, including dispensing
Operating results by business segment
Net revenue	(47,211)	(94,095)
Elimination | Commercial | Risk-based, non-EAP
Operating results by business segment
Net revenue	(114)	(114)
Elimination | Commercial | ASO
Operating results by business segment
Net revenue	(37)	(182)
Elimination | Non-government
Operating results by business segment
Net revenue	$ (47,362)	$ (94,391)