Pension and Other Postretirement Benefits - Amounts Recognized in AOCI and Disaggregated Information (Details) - USD ($) $ in Millions	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Defined Benefit Plan Disclosure [Line Items]
Total recognized in accumulated other comprehensive income	$ 64.5	$ 57.5
Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Accumulated actuarial loss	95.8	84.1
Prior service cost	0.9	1.2
Total recognized in accumulated other comprehensive income	96.7	85.3
Assets Exceed ABO
Projected benefit obligation	291.3	280.1
Accumulated benefit obligation	281.5	269.1
Fair value of plan assets	284.2	270.4
ABO Exceed Assets
Projected benefit obligation	79.6	80.0
Accumulated benefit obligation	79.4	79.8
Fair value of plan assets	33.9	37.9
Total
Projected benefit obligation	370.9	360.1	$ 330.2
Accumulated benefit obligation	360.9	348.9
Fair value of plan assets	318.1	308.3	288.3
Postretirement Benefits Other than Pensions
Defined Benefit Plan Disclosure [Line Items]
Accumulated actuarial loss	4.9	5.8
Prior service cost	(0.4)	(0.5)
Total recognized in accumulated other comprehensive income	4.5	5.3
Total
Projected benefit obligation	40.7	40.5	40.7
Fair value of plan assets	$ 0.0	$ 0.0	$ 0.0