Savings, Pension and Other Postretirement Employee Benefit Plans - Pre-tax Components of Net Periodic Cost (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Pension Benefit Plans
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 2.4	$ 1.8	$ 2.1
Interest cost	12.4	12.0	13.1
Expected return on plan assets	(16.5)	(17.0)	(18.8)
Amortization of prior service cost (credit)	0.0	0.0	0.0
Amortization of actuarial loss	7.3	10.1	9.9
Defined Benefit Plan, Benefit Obligation, (Increase) Decrease for Settlement	0.0	0.0	0.0
Net periodic cost	5.6	6.9	6.3
Other Postretirement Employee Benefit Plans
Defined Benefit Plan Disclosure [Line Items]
Service cost	0.1	0.1	0.2
Interest cost	2.8	2.4	2.7
Expected return on plan assets	0.0	0.0	0.0
Amortization of prior service cost (credit)	0.0	(1.7)	(1.5)
Amortization of actuarial loss	(0.3)	(0.9)	(6.6)
Defined Benefit Plan, Benefit Obligation, (Increase) Decrease for Settlement	0.0	0.0	0.0
Net periodic cost	$ 2.6	$ (0.1)	$ (5.2)