Acquisitions Cash Paid for Acquisitions (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Cash Paid for Acquisitions [Abstract]
Cost of investment: current-year acquisitions 1	$ 0.6	$ 2,323.4	$ 36.8
Cost of investment: prior-year acquisitions	15.8	33.9	54.6
Less: net cash acquired	0.0	(13.8)	(7.1)
Total cost of investment	16.4	2,343.5	84.3
Operating payments	9.3	19.4	47.1
Total cash paid for acquisitions	25.7	2,362.9	131.4
Acquisitions, net of cash acquired	0.6	2,309.8	30.6
Payments for previous acquisition, financing cash flows	$ 15.8	$ 33.7	$ 53.7