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, 2019	Dec. 31, 2018	Dec. 31, 2017
Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Net periodic benefit expense	$ 3.7	$ 7.7	$ 7.0
Accumulated actuarial gain (loss)	7.7	4.2	10.1
Prior service cost (credit)	0.2	(0.1)	(0.1)
Total recognized in other comprehensive income (loss)	7.9	4.1	10.0
Total recognized in net periodic benefit cost and other comprehensive income (loss)	11.6	11.8	17.0
Postretirement Benefits Other than Pensions
Defined Benefit Plan Disclosure [Line Items]
Net periodic benefit expense	3.6	3.1	2.7
Accumulated actuarial gain (loss)	(1.5)	0.1	3.7
Prior service cost (credit)	0.0	0.2	0.2
Total recognized in other comprehensive income (loss)	(1.5)	0.3	3.9
Total recognized in net periodic benefit cost and other comprehensive income (loss)	$ 2.1	$ 3.4	$ 6.6