Fair Value on a Recurring Basis (Details) - USD ($) $ in Millions		6 Months Ended
		Jun. 30, 2018	Dec. 31, 2017
Fair value assets and liabilities measured on recurring basis
Contingent acquisition liability, total change		$ 0.2
Level 1
Fair value assets and liabilities measured on recurring basis
Cash equivalents		111.8	$ 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]	147.2	147.0
Fair Value, Total [Member]
Fair value assets and liabilities measured on recurring basis
Cash equivalents		111.8	201.6
Contingent acquisition obligations	[1]	$ 147.2	$ 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 increase in this balance of $0.2 from December 31, 2017 to June 30, 2018 is primarily due to new acquisitions and option exercises offset by payments. The amounts payable within the next twelve months are classified in accrued liabilities; any amounts payable thereafter are classified in other non-current liabilities.