Pension and Other Post-Retirement Benefits - Components of Net Periodic Cost (Benefit) (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Defined Benefit Pension [Member]
Components of pension and other post retirement benefit costs
Service cost	$ 2.2	$ 1.7	$ 4.1	$ 3.3
Interest cost	3.7	3.9	7.4	7.7
Expected return 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
Amortization of unrecognized prior service credit	0.0	0.0	0.0	0.0
Unrecognized net actuarial loss (gain)	2.6	2.4	5.1	4.7
Net periodic (benefit) cost	3.0	2.1	5.5	3.8
Retiree Health and Life [Member]
Components of pension and other post retirement benefit costs
Service cost	0.0	0.1	0.1	0.1
Interest cost	0.3	0.3	0.5	0.5
Expected return on plan assets	0.0	0.0	0.0	0.0
Amortization of unrecognized prior service credit	(0.1)	(0.1)	(0.1)	(0.1)
Unrecognized net actuarial loss (gain)	0.0	(0.1)	0.0	(0.2)
Net periodic (benefit) cost	$ 0.2	$ 0.2	$ 0.5	$ 0.3