Fair Value Measurement (Narrative) (Details) - USD ($) $ in Millions	Sep. 30, 2018	Dec. 31, 2017	Sep. 30, 2017	Dec. 31, 2016
Fair Value Disclosures [Abstract]
Liability	$ 616.2	$ 1,168.2	$ 1,186.9	$ 1,058.0
Contingent consideration payable	82.3
Contingent consideration payable	533.9
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Joint venture net written option	22.5	11.5
Level 3 | Recurring Basis
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Joint venture net written option	$ 48.0	$ 40.0