Retirement Plans & Other Postretirement Benefits - Funded Status of Plans (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value of plan assets, end of period	$ 316.1	$ 304.5
Pension Benefits
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Projected benefit obligation, end of period	(366.5)	(350.4)	$ (348.3)
Fair value of plan assets, end of period	316.1	304.5	296.9
Funded (unfunded) status of plan	(50.2)	(45.8)
Accrued liabilities	1.1	1.2
Deferred credits & other liabilities	49.1	44.6
Pension Benefits | Qualified Plans
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Projected benefit obligation, end of period	(343.4)	(329.7)
Fair value of plan assets, end of period	316.1	304.5
Funded (unfunded) status of plan	(27.3)	(25.2)
Other Benefits
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Projected benefit obligation, end of period	(40.0)	(40.5)	(43.5)
Fair value of plan assets, end of period	0.0	0.0	$ 0.0
Funded (unfunded) status of plan	(40.0)	(40.5)
Accrued liabilities	4.1	4.5
Deferred credits & other liabilities	35.9	36.0
Other Benefits | Qualified Plans
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Projected benefit obligation, end of period	(40.0)	(40.5)
Fair value of plan assets, end of period	0.0	0.0
Funded (unfunded) status of plan	(40.0)	(40.5)
SERP
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Projected benefit obligation, end of period	$ (22.9)	$ (20.6)