Fair Value on a Recurring Basis (Details) - USD ($) $ in Millions		9 Months Ended
		Sep. 30, 2018	Dec. 31, 2017
Fair value assets and liabilities measured on recurring basis
Contingent acquisition liability, total change		$ (7.3)
Level 1
Fair value assets and liabilities measured on recurring basis
Cash equivalents		96.9	$ 201.6
Contingent acquisition obligations	[1]	0.0	0.0
Level 2
Fair value assets and liabilities measured on recurring basis
Cash equivalents		0.0	0.0
Contingent acquisition obligations	[1]	0.0	0.0
Level 3
Fair value assets and liabilities measured on recurring basis
Cash equivalents		0.0	0.0
Contingent acquisition obligations	[1]	139.7	147.0
Fair Value, Total [Member]
Fair value assets and liabilities measured on recurring basis
Cash equivalents		96.9	201.6
Contingent acquisition obligations	[1]	$ 139.7	$ 147.0

[1] Contingent acquisition obligations includes deferred acquisition payments and unconditional obligations to purchase additional noncontrolling equity shares of consolidated subsidiaries. Fair value measurement of the obligations is based upon actual and projected operating performance targets as specified in the related agreements. The decrease in this balance of $7.3 from December 31, 2017 to September 30, 2018 is primarily due to payments, partially offset by new acquisitions and option exercises. The amounts payable within the next twelve months are classified in accrued liabilities; any amounts payable thereafter are classified in other non-current liabilities.