Acquisitions Cash Paid for Acquisitions (Details) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Cash Paid for Acquisitions [Abstract]
Cost of investment: current-year acquisitions		$ 65.7	$ 37.8	$ 97.3
Cost of investment: prior-year acquisitions		40.7	53.1	14.0
Less: net cash acquired		(13.6)	(9.2)	(29.9)
Total cost of investment		92.8	81.7	81.4
Operating expense	[1]	19.1	18.4	3.4
Total cash paid for acquisitions	[2]	111.9	100.1	84.8
Payments for Previous Acquisitions, Financing Cash Flows		40.8	53.1	13.6
Payments for Initial Acquisitions, Investing Cash Flows		$ 52.0	$ 28.6	$ 67.8

[1] Represents cash payments made that were either in excess of the initial value of contingent payments or contingent upon the future employment of the former owners of the acquired companies and are recorded in the operating section of the Consolidated Statements of Cash Flows.

[2] Of the total cash paid for acquisitions, $52.0, $28.6 and $67.8 for the years ended December 31, 2016, 2015 and 2014, respectively, are classified under the investing section of the Consolidated Statements of Cash Flows as acquisitions, net of cash acquired. These amounts relate to initial payments for new transactions. Of the total cash paid for acquisitions, $40.8, $53.1 and $13.6 for the years ended December 31, 2016, 2015 and 2014, respectively, are classified under the financing section of the Consolidated Statements of Cash Flows as acquisition-related payments. These amounts relate to deferred payments and increases in our ownership interest for prior acquisitions.