Retirement Plans & Other Postretirement Benefits - Components of Net Periodic Benefit Cost (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 6.5	$ 7.0	$ 7.9
Interest cost	13.7	14.7	14.6
Expected return on plan assets	(21.0)	(22.8)	(22.5)
Amortization of prior service cost (benefit)	0.4	0.4	0.7
Amortization of actuarial loss	7.4	7.2	8.5
Settlement charge	2.1	0.0	0.6
Net periodic benefit cost (benefit)	9.1	6.5	9.8
Other Benefits
Defined Benefit Plan Disclosure [Line Items]
Service cost	0.2	0.3	0.4
Interest cost	1.5	1.7	2.0
Expected return on plan assets	0.0	0.0	0.0
Amortization of prior service cost (benefit)	(2.4)	(2.9)	(3.0)
Amortization of actuarial loss	0.0	0.0	0.7
Settlement charge	0.0	0.0	0.0
Net periodic benefit cost (benefit)	$ (0.7)	$ (0.9)	$ 0.1