Summary of Accounting Policies - Allowance for Doubtful Accounts (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Activity in allowance for doubtful accounts
Balance at the Beginning of the period	$ 6.5	$ 3.9	$ 4.7
(Decrease) increase to reserve	(0.5)	1.2	0.1
Amounts acquired	0.0	2.0	0.4
Amounts written off	(0.9)	(0.6)	(1.3)
Balance at the end of the period	$ 5.1	$ 6.5	$ 3.9