Pension and Other Post-Retirement Benefits - Components of Net Periodic Cost (Benefit) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Defined Benefit Pension [Member]
Components of pension and other post retirement benefit costs
Service cost	$ 6.5	$ 6.1	$ 7.4
Interest cost	15.4	15.3	19.6
Expected return on plan assets	(24.0)	(25.6)	(25.8)
Settlement accounting adjustment	0.2	6.1	7.3
Amortization of unrecognized prior service credit	0.0	(1.0)	(1.0)
Unrecognized net actuarial loss (gain)	9.3	10.5	14.8
Net periodic (benefit) cost	7.4	11.4	22.3
Retiree Health and Life [Member]
Components of pension and other post retirement benefit costs
Service cost	0.2	0.2	0.2
Interest cost	1.1	0.9	1.3
Expected return on plan assets	0.0	0.0	0.0
Settlement accounting adjustment	0.0	0.0	1.7
Amortization of unrecognized prior service credit	(0.2)	(0.2)	(0.1)
Unrecognized net actuarial loss (gain)	(0.3)	(0.3)	(0.2)
Net periodic (benefit) cost	$ 0.8	$ 0.6	$ 2.9