Summary of Significant Accounting Policies - Revenues (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Net revenues
Managed Care Revenue	$ 2,700.0	$ 2,300.0	$ 2,700.0
Fee-For-Service, Fixed Fee and Cost-Plus Contracts Revenue	604.3	503.2	342.0
Rebate Revenues	91.9	85.4	88.7
PBM Revenue	1,700.0	1,500.0	1,200.0
Dispensing Revenue	206.0	221.8	$ 211.6
Medicare Part D Revenues	511.0	272.8
Co-payments	70.6	31.0
Medicare Part D Receivables	$ 131.5	$ 117.5