Acquisitions Cash Paid for Acquisitions (Details) - USD ($) $ in Millions	6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018
Cash Paid for Acquisitions [Abstract]
Cost of investment: current-year acquisitions	$ 0.6	$ 8.7
Cost of investment: prior-year acquisitions	13.0	16.2
Less: net cash acquired	0.0	(0.4)
Total cost of investment	13.6	24.5
Operating payments	9.1	18.2
Total cash paid for acquisitions	$ 22.7	$ 42.7