Pension and Other Postretirement Benefits - Net Periodic Benefit Cost (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Defined Benefit Pension Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 4.3	$ 4.4	$ 4.6
Interest cost	12.4	13.3	13.9
Expected return on plan assets	(21.1)	(19.0)	(18.8)
Amortization of prior service costs	0.1	0.1	0.1
Amortization of net loss	5.2	4.7	4.7
Curtailment (benefit) cost	0.0	0.0	0.0
Net periodic benefit cost	0.9	3.5	4.5
Other Postretirement Benefit Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	0.2	0.8	1.0
Interest cost	4.2	5.3	5.6
Expected return on plan assets	0.0	0.0	0.0
Amortization of prior service costs	(7.3)	(3.7)	(2.7)
Amortization of net loss	4.0	3.9	3.5
Curtailment (benefit) cost	0.0	(1.4)	0.0
Net periodic benefit cost	$ 1.2	$ 4.9	$ 7.4