Employee Benefit Plans - Components of Net Periodic Benefit Expense (Details) $ in Millions	12 Months Ended
	Dec. 31, 2017 USD ($) plan	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)
Defined Benefit Plan
Service cost	$ 1.0	$ 0.9	$ 0.9
Interest cost	$ 0.9	1.0	0.7
Defined Benefit Pension Plans | Germany
Defined Benefit Plan
Number of defined benefit plans | plan	3
Service cost	$ 1.0	0.9	0.9
Interest cost	0.9	1.0	0.7
Amortization of net actuarial gains		(0.3)
Expected return on plan assets		(0.1)	(0.1)
Net periodic benefit expense	$ 1.9	$ 1.5	$ 1.5