Pension and Postretirement Plans - Amounts Recognized in AOCI (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Pension Plan
Prior service cost recognized:
Reclassification adjustments	$ 0.0	$ 0.1
Actuarial (loss) gain recognized:
Reclassification adjustments	21.5	19.1
Actuarial gain (loss) arising during the period	11.0	5.7
Prior service benefit	0.0
Actuarial loss	(17.6)
Total	17.6
Other Postretirement Benefits Plan
Prior service cost recognized:
Reclassification adjustments	(4.5)	(14.7)
Actuarial (loss) gain recognized:
Reclassification adjustments	4.7	4.9
Actuarial gain (loss) arising during the period	(7.4)	$ 4.5
Prior service benefit	3.1
Actuarial loss	(4.6)
Total	$ 1.5