Pension and Other Postretirement Benefits - Other Changes in Plan Assets and Benefit Obligations Recognized in OCI (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Net periodic benefit expense	$ 7.0	$ 9.5	$ (8.4)
Accumulated actuarial gain (loss)	10.1	11.7	(7.1)
Prior service cost (credit)	(0.1)	(0.3)	(0.3)
Total recognized in other comprehensive income	10.0	11.4	(7.4)
Total recognized in net periodic benefit cost and other comprehensive income	17.0	20.9	(15.8)
Postretirement Benefits Other than Pensions
Defined Benefit Plan Disclosure [Line Items]
Net periodic benefit expense	2.7	3.3	3.4
Accumulated actuarial gain (loss)	3.7	(0.9)	1.1
Prior service cost (credit)	0.2	0.1	0.2
Total recognized in other comprehensive income	3.9	(0.8)	1.3
Total recognized in net periodic benefit cost and other comprehensive income	$ 6.6	$ 2.5	$ 4.7