Fair Value on a Recurring Basis (Details) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2018	Dec. 31, 2017
Fair value assets and liabilities measured on recurring basis
Cash equivalents		$ 132.1	$ 201.6
Contingent acquisition obligations	[1]	148.4	147.0
Contingent acquisition obligation, total change		1.4
Contingent acquisition obligation, payments		(54.0)
Contingent acquisition obligation, additions		61.8
Level 1
Fair value assets and liabilities measured on recurring basis
Cash equivalents		132.1	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]	$ 148.4	$ 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 $1.4 from December 31, 2017 to December 31, 2018 is primarily due to acquisitions and exercised put options of $61.8, partially offset by payments of $54.0. The amounts payable within the next twelve months are classified in accrued liabilities; any amounts payable thereafter are classified in other non-current liabilities.