Pension and Other Post-Retirement Benefits - Pension Obligations, Plan Assets, and Other Post-retirement Obligations (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Mar. 31, 2018	Dec. 31, 2017
Accumulative other comprehensive loss:
After-tax amount recognized in accumulated other comprehensive loss	$ 96.2		$ 96.2		$ 98.1	$ 83.6
Defined Benefit Pension [Member]
Change in Benefit Obligation
Service cost	2.2	$ 1.7	4.1	$ 3.3
Interest cost	3.7	3.9	7.4	7.7
Accumulative other comprehensive loss:
Defined Benefit Plan, Expected Return (Loss) on Plan Assets	(5.5)	(6.0)	(11.1)	(12.0)
Defined Benefit Plan, Net Periodic Benefit Cost (Credit), Gain (Loss) Due to Settlement		0.1		0.1
Defined Benefit Plan, Amortization of Prior Service Cost (Credit)	0.0	0.0	0.0	0.0
Defined Benefit Plan, Amortization of Gain (Loss)	2.6	2.4	5.1	4.7
Defined Benefit Plan, Net Periodic Benefit Cost (Credit)	3.0	2.1	5.5	3.8
Retiree Health and Life [Member]
Change in Benefit Obligation
Service cost	0.0	0.1	0.1	0.1
Interest cost	0.3	0.3	0.5	0.5
Accumulative other comprehensive loss:
Defined Benefit Plan, Expected Return (Loss) on Plan Assets	0.0	0.0	0.0	0.0
Defined Benefit Plan, Amortization of Prior Service Cost (Credit)	(0.1)	(0.1)	(0.1)	(0.1)
Defined Benefit Plan, Amortization of Gain (Loss)	0.0	(0.1)	0.0	(0.2)
Defined Benefit Plan, Net Periodic Benefit Cost (Credit)	$ 0.2	$ 0.2	$ 0.5	$ 0.3