Employee Benefit Plans - Net Periodic Benefit Cost (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Retirement Benefits [Abstract]
Service cost	$ 2.4	$ 2.2	$ 2.5
Interest cost	4.2	4.1	4.3
Expected return on plan assets	(5.9)	(5.8)	(5.9)
Amortization of actuarial losses	1.4	0.3	1.1
Net periodic cost	$ 2.1	$ 0.8	$ 2.0