Pension and Postretirement Plans - Components of Net Periodic Cost (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Jun. 30, 2015	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Pension Plan
Defined Benefit Plan Disclosure [Line Items]
Service cost		$ 0.0	$ 0.0	$ 0.3
Interest cost on projected benefit obligation		19.4	19.3	19.0
Expected return on plan assets		(26.0)	(27.3)	(29.2)
Amortization of:
Prior service cost (benefit)		0.0	0.1	0.1
Actuarial loss		17.5	19.1	24.9
Pension settlement charges		4.0	0.0	0.0
Curtailment loss	$ 0.3	0.0	0.0	0.3
Pension/postretirement cost (benefit)		14.9	11.2	15.4
Other Postretirement Benefits Plan
Defined Benefit Plan Disclosure [Line Items]
Service cost		0.2	0.3	0.3
Interest cost on projected benefit obligation		3.2	3.3	3.3
Expected return on plan assets		0.0	0.0	0.0
Amortization of:
Prior service cost (benefit)		(4.5)	(14.7)	(15.4)
Actuarial loss		4.7	4.9	5.4
Pension settlement charges		0.0	0.0	0.0
Curtailment loss		0.0	0.0	0.0
Pension/postretirement cost (benefit)		$ 3.6	$ (6.2)	$ (6.4)