Retirement Plans & Other Postretirement Benefits - Amounts in AOCI (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Pension Benefits
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Prior service cost	$ 2.3	$ 1.4	$ 1.8	$ 2.0
Unamortized actuarial loss	110.3	114.6	110.1	111.7
Sub total	112.6	116.0
Less: Regulatory asset deferral	(110.2)	(113.6)
AOCI before taxes	2.4	2.4
Other Benefits
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Prior service cost	(8.8)	(11.2)	(14.1)	(17.1)
Unamortized actuarial loss	1.2	0.0	$ 1.6	$ 10.9
Sub total	(7.6)	(11.2)
Less: Regulatory asset deferral	7.4	11.0
AOCI before taxes	$ (0.2)	$ (0.2)