Postretirement Benefit Plans - Net Periodic Benefit Cost (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Pension Plan [Member]
Components of net periodic benefit cost:
Defined Benefit Plan, Actuarial Gain (Loss), Immediate Recognition as Component in Net Periodic Benefit (Cost) Credit	$ 23.2	$ 60.9	$ 21.1
Postretirement Benefit Plans [Member]
Components of net periodic benefit cost:
Service cost	(0.1)	(0.3)	(0.4)
Interest cost	(9.1)	(11.0)	(10.9)
Expected return on plan assets	(5.6)	(6.3)	(7.3)
Amortization of prior service (credit) cost	1.0	(1.0)	(0.8)
Defined Benefit Plan, Actuarial Gain (Loss), Immediate Recognition as Component in Net Periodic Benefit (Cost) Credit	4.0	(4.5)	(1.0)
Defined Benefit Plan, Net Periodic Benefit Cost (Credit), Gain (Loss) Due to Settlement and Curtailment	0.0	0.1	0.0
Net periodic benefit cost	$ (1.4)	$ 10.6	$ 5.8