Pension and Other Postretirement Benefits - Components of Net Periodic Benefit Cost (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Defined Benefit Plan Disclosure [Line Items]
Curtailment gain	$ (1.6)
Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Service cost		$ 5.0	$ 6.7	$ 5.5
Interest cost		16.2	15.8	15.0
Expected return on plan assets		(21.1)	(21.0)	(19.9)
Recognized net actuarial loss		4.9	5.2	5.6
Amortization of prior service cost (credit)		0.2	0.2	0.2
Curtailment gain		(1.6)	0.0	0.0
Amount of settlement loss recognized				0.6
Net periodic benefit cost		3.7	7.7	7.0
Postretirement Benefits Other than Pensions
Defined Benefit Plan Disclosure [Line Items]
Service cost		1.2	1.1	1.2
Interest cost		1.5	1.4	1.4
Expected return on plan assets		0.0	0.0	0.0
Recognized net actuarial loss		0.9	0.8	0.3
Amortization of prior service cost (credit)		0.0	(0.2)	(0.2)
Curtailment gain		0.0	0.0	0.0
Amount of settlement loss recognized		0.0	0.0	0.0
Net periodic benefit cost		$ 3.6	$ 3.1	$ 2.7