Summary of Accounting Policies - Allowance for Doubtful Accounts (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Activity in allowance for doubtful accounts
Balance at the Beginning of the period	$ 4.0	$ 4.7	$ 4.8
Provision charged to expense	1.3	0.3	0.1
Amounts acquired	2.0	0.4	1.5
Amounts written off, net of recoveries	(0.7)	(1.4)	(1.7)
Balance at the end of the period	$ 6.6	$ 4.0	$ 4.7