Post Retirement Benefit Plans - Changes in Benefit Obligations and Plan Assets (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Change in benefit obligation:
Benefit obligation at beginning of year	$ 210.1	$ 199.2
Service cost	7.8	6.8	$ 7.2
Interest cost	1.7	2.2	2.5
Plan participant contributions	4.1	4.0
Benefits paid	(3.6)	(7.1)
Actuarial loss (gain)	(3.6)	11.1
Premiums paid	(1.5)	(1.4)
Impact of foreign currency exchange rates	13.0	(4.7)
Benefit obligation at end of year	228.0	210.1	199.2
Change in plan assets:
Fair value of plan assets at beginning of year	105.9	106.1
Return on plan assets	5.0	1.3
Plan participant and employer contributions	9.5	9.2
Benefits paid	(3.6)	(7.1)
Premiums paid	(1.5)	(1.4)
Impact of foreign currency exchange rates	5.0	(2.2)
Fair value of plan assets at end of year	120.3	105.9	$ 106.1
Net under funded status	(107.7)	(104.2)
Accumulated benefit obligation	$ 217.2	$ 199.9