Acquisitions Cash Paid for Acquisitions (Details) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Cash Paid for Acquisitions [Abstract]
Cost of investment: current-year acquisitions		$ 36.8	$ 65.7	$ 37.8
Cost of investment: prior-year acquisitions		54.6	40.7	53.1
Less: net cash acquired		(7.1)	(13.6)	(9.2)
Total cost of investment		84.3	92.8	81.7
Operating payments	[1]	47.1	19.1	18.4
Total cash paid for acquisitions	[2]	131.4	111.9	100.1
Acquisitions, net of cash acquired, investing cash flows		30.6	52.0	28.6
Payments for previous acquisition, financing cash flows		$ 53.7	$ 40.8	$ 53.1

[1] Represents cash payments for amounts that have been recognized in operating expenses since the date of acquisition either relating to adjustments to estimates in excess of the initial value of contingent payments recorded or were contingent upon the future employment of the former owners of the acquired companies. Amounts are reflected in the operating section of the Consolidated Statements of Cash Flows.

[2] Of the total cash paid for acquisitions, $30.6, $52.0 and $28.6 for the years ended December 31, 2017, 2016 and 2015, 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, $53.7, $40.8 and $53.1 for the years ended December 31, 2017, 2016 and 2015, 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.