Employee Benefit Plans - Change in Level 3 Plan Assets (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Change in fair value of the pension assets valued using significant unobservable inputs (Level 3)
Fair value of plan assets at beginning of year	$ 99.3	$ 104.5
Fair value of assets at end of year	115.5	99.3
Level 3
Change in fair value of the pension assets valued using significant unobservable inputs (Level 3)
Fair value of plan assets at beginning of year	10.9	12.9
Unrealized gain (loss) on asset still held	(6.7)	(2.0)
Fair value of assets at end of year	$ 4.2	$ 10.9