Fair Value Measurement (Narrative) (Details) - USD ($) $ in Millions	Jun. 30, 2018	Dec. 31, 2017	Jun. 30, 2017	Dec. 31, 2016
Fair Value Disclosures [Abstract]
Liability	$ 1,209.2	$ 1,168.2	$ 1,190.3	$ 1,058.0
Contingent consideration payable	734.7
Contingent consideration payable	474.5
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Joint venture net written option	56.4	11.5
Level 3 | Recurring Basis
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Joint venture net written option	$ 37.0	$ 40.0