Acquisitions Cash Paid for Acquisitions (Details) - USD ($) $ in Millions		9 Months Ended
		Sep. 30, 2018	Sep. 30, 2017
Cash Paid for Acquisitions [Abstract]
Cost of investment: current-year acquisitions		$ 12.3	$ 28.1
Cost of investment: prior-year acquisitions		30.6	50.0
Less: net cash acquired		(0.5)	(6.4)
Total cost of investment		42.4	71.7
Operating payments	[1]	18.3	37.5
Total cash paid for acquisitions	[2]	60.7	109.2
Payments for initial acquisitions, Investing Cash Flows		12.0	22.6
Payments for previous acquisitions, Financing Cash Flows		$ 30.4	$ 49.1

[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 unaudited Consolidated Statements of Cash Flows.

[2] Of the total cash paid for acquisitions, $12.0 and $22.6 for the nine months ended September 30, 2018 and 2017, respectively, are classified under the investing section of the unaudited 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, $30.4 and $49.1 for the nine months ended September 30, 2018 and 2017, respectively, are classified under the financing section of the unaudited Consolidated Statements of Cash Flows as acquisition-related payments. These amounts relate to deferred payments and increases in our ownership interest for prior acquisitions.