Fair Value Measurement (Narrative) (Details) - USD ($) $ in Millions	Mar. 31, 2018	Dec. 31, 2017	Mar. 31, 2017	Dec. 31, 2016
Fair Value Disclosures [Abstract]
Liability	$ 1,179.6	$ 1,168.2	$ 1,051.2	$ 1,058.0
Contingent consideration payable	700.8
Contingent consideration payable	478.8
Fair value | Recurring Basis
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Joint venture net written option	32.0	40.0
Level 3 | Recurring Basis
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Joint venture net written option	$ 32.0	$ 40.0