Acquisitions Cash Paid for Acquisitions (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Cash Paid for Acquisitions [Abstract]
Cost of investment: current-year acquisitions 1	$ 2,323.4	$ 36.8	$ 65.7
Cost of investment: prior-year acquisitions	33.9	54.6	40.7
Less: net cash acquired	(13.8)	(7.1)	(13.6)
Total cost of investment	2,343.5	84.3	92.8
Operating payments	19.4	47.1	19.1
Total cash paid for acquisitions	2,362.9	131.4	111.9
Acquisitions, net of cash acquired	2,309.8	30.6	52.0
Payments for previous acquisition, financing cash flows	$ 33.7	$ 53.7	$ 40.8