Pension and Postretirement Plans - Benefit Obligation and Funded Status (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets at January 1,	$ 381.0
Fair value of plan assets at December 31,	399.6	$ 381.0
Pension Plan
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Benefit obligation at January 1,	505.6	530.5
Service cost	0.0	0.0	$ 0.3
Interest cost	19.4	19.3	19.0
Actuarial loss (gain)	28.1	(2.7)
Benefits paid	(38.6)	(41.5)
Retiree drug subsidy received	0.0	0.0
Transfer of special death benefit	(14.0)	0.0
Settlements	(11.3)	0.0
Other	0.0	0.0
Benefit obligation at December 31,	489.2	505.6	530.5
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets at January 1,	372.3	378.1
Actual return (loss) on plan assets	64.8	30.3
Employer contributions	4.6	5.4
Retiree drug subsidy received	0.0	0.0
Benefits paid	(38.6)	(41.5)
Settlements	(11.0)	0.0
Fair value of plan assets at December 31,	392.1	372.3	378.1
Unfunded status	(97.1)	(133.3)
Other Postretirement Benefits Plan
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Benefit obligation at January 1,	82.6	93.1
Service cost	0.2	0.3	0.3
Interest cost	3.2	3.3	3.3
Actuarial loss (gain)	7.6	(4.7)
Benefits paid	(11.6)	(13.1)
Retiree drug subsidy received	0.2	0.6
Transfer of special death benefit	14.0	0.0
Settlements	0.0	0.0
Other	2.4	3.1
Benefit obligation at December 31,	98.6	82.6	93.1
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets at January 1,	8.7	10.3
Actual return (loss) on plan assets	0.2	0.3
Employer contributions	10.0	10.6
Retiree drug subsidy received	0.2	0.6
Benefits paid	(11.6)	(13.1)
Settlements	0.0	0.0
Fair value of plan assets at December 31,	7.5	8.7	$ 10.3
Unfunded status	$ (91.1)	$ (73.9)