General – Revenue Recognition (Details) $ in Thousands	3 Months Ended
	Mar. 31, 2018 USD ($)
Operating results by business segment
Net revenue	$ 1,805,077
Transferred at a point in time
Operating results by business segment
Net revenue	585,314
Transferred over time
Operating results by business segment
Net revenue	1,219,763
PBM, including dispensing
Operating results by business segment
Net revenue	486,908
Medicare Part D
Operating results by business segment
Net revenue	98,406
PBA
Operating results by business segment
Net revenue	33,546
Formulary management
Operating results by business segment
Net revenue	20,377
Other service lines
Operating results by business segment
Net revenue	1,084
Commercial | Risk-based, non-EAP
Operating results by business segment
Net revenue	234,391
Commercial | EAP risk-based
Operating results by business segment
Net revenue	12,421
Commercial | ASO
Operating results by business segment
Net revenue	59,724
Government
Operating results by business segment
Net revenue	1,086,269
Government | Risk-based, non-EAP
Operating results by business segment
Net revenue	752,490
Government | EAP risk-based
Operating results by business segment
Net revenue	82,237
Government | ASO
Operating results by business segment
Net revenue	23,493
Non-government
Operating results by business segment
Net revenue	718,808
Healthcare
Operating results by business segment
Net revenue	1,157,601
Healthcare | Transferred over time
Operating results by business segment
Net revenue	1,157,601
Healthcare | Commercial | Risk-based, non-EAP
Operating results by business segment
Net revenue	234,391
Healthcare | Commercial | EAP risk-based
Operating results by business segment
Net revenue	12,421
Healthcare | Commercial | ASO
Operating results by business segment
Net revenue	52,569
Healthcare | Government
Operating results by business segment
Net revenue	858,220
Healthcare | Government | Risk-based, non-EAP
Operating results by business segment
Net revenue	752,490
Healthcare | Government | EAP risk-based
Operating results by business segment
Net revenue	82,237
Healthcare | Government | ASO
Operating results by business segment
Net revenue	23,493
Healthcare | Non-government
Operating results by business segment
Net revenue	299,381
Pharmacy Management
Operating results by business segment
Net revenue	694,505
Pharmacy Management | Transferred at a point in time
Operating results by business segment
Net revenue	632,198
Pharmacy Management | Transferred over time
Operating results by business segment
Net revenue	62,307
Pharmacy Management | PBM, including dispensing
Operating results by business segment
Net revenue	533,792
Pharmacy Management | Medicare Part D
Operating results by business segment
Net revenue	98,406
Pharmacy Management | PBA
Operating results by business segment
Net revenue	33,546
Pharmacy Management | Formulary management
Operating results by business segment
Net revenue	20,377
Pharmacy Management | Other service lines
Operating results by business segment
Net revenue	1,084
Pharmacy Management | Commercial | ASO
Operating results by business segment
Net revenue	7,300
Pharmacy Management | Government
Operating results by business segment
Net revenue	228,049
Pharmacy Management | Non-government
Operating results by business segment
Net revenue	466,456
Elimination
Operating results by business segment
Net revenue	(47,029)
Elimination | Transferred at a point in time
Operating results by business segment
Net revenue	(46,884)
Elimination | Transferred over time
Operating results by business segment
Net revenue	(145)
Elimination | PBM, including dispensing
Operating results by business segment
Net revenue	(46,884)
Elimination | Commercial | ASO
Operating results by business segment
Net revenue	(145)
Elimination | Non-government
Operating results by business segment
Net revenue	$ (47,029)