Employee Benefit Plans - Components Of Net Period Benefit Cost (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Pension Benefits
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Service cost	$ 0.6	$ 1.0	$ 1.7
Interest cost	19.3	20.7	21.3
Expected return on plan assets	(27.7)	(31.4)	(32.7)
Mark-to-market actuarial net losses (gains)	5.0	(7.8)	15.2
Net periodic (benefit) cost	(2.8)	(17.5)	5.5
Health Care Benefits
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Service cost	0.0	0.0	0.0
Interest cost	0.4	0.5	0.6
Expected return on plan assets	0.0	0.0	0.0
Mark-to-market actuarial net losses (gains)	(1.7)	(0.6)	(3.6)
Net periodic (benefit) cost	$ (1.3)	$ (0.1)	$ (3.0)