Pensions and Other Postretirement Benefits - Pension and Postretirement Expenses (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Pension Benefits
Components of Net Periodic Cost:
Service Cost	$ 4.6	$ 1.9	$ 9.2	$ 4.5
Interest Cost	10.5	10.6	21.0	21.2
Administrative Expenses	0.1	0.0	0.2	0.0
Expected Return on Plan Assets	(15.9)	(15.9)	(31.9)	(31.9)
Amortization:
Prior Service Cost (Credit)	0.1	0.2	0.2	0.3
Actuarial Loss (Gain)	1.4	1.5	2.8	3.2
Net Periodic Cost (Benefit)	0.8	(1.7)	1.5	(2.7)
Postretirement Health Care Benefits
Components of Net Periodic Cost:
Service Cost	0.1	0.2	0.3	0.4
Interest Cost	0.3	0.4	0.6	0.7
Administrative Expenses	0.0	0.0	0.0	0.0
Expected Return on Plan Assets	0.0	0.0	0.0	0.0
Amortization:
Prior Service Cost (Credit)	0.0	0.0	(0.1)	(0.1)
Actuarial Loss (Gain)	(0.4)	(0.6)	(0.9)	(1.1)
Net Periodic Cost (Benefit)	$ 0.0	$ 0.0	$ (0.1)	$ (0.1)