Regulatory Assets and Liabilities (Details) (USD $) In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Regulatory Assets [Line Items]
Regulatory assets earnings return based on short-term rates	$ 115.1
Remaining Amounts of Regulatory Assets for which No Return on Investment During Recovery Period is Provided	12.8
Regulatory Assets
Regulatory Assets	1,271.2	1,108.5
Escrowed PTF [Member]
Regulatory Assets
Regulatory Assets	66.6	49.3
Deferred unrecognized pension costs [Member]
Regulatory Assets
Regulatory Assets	629.5	537.6
Deferred income tax related [Member]
Regulatory Assets
Regulatory Assets	176.0	169.5
Escrowed electric transmission costs [Member]
Regulatory Assets
Regulatory Assets	146.0	126.8
Escrowed conservation [Member]
Regulatory Assets
Regulatory Assets	58.0	66.9
Deferred plant related -- capital lease [Member]
Regulatory Assets
Regulatory Assets	42.3	56.5
Deferred environmental costs [Member]
Regulatory Assets
Regulatory Assets	45.9	47.0
Other, net [Member]
Regulatory Assets
Regulatory Assets	$ 106.9	$ 54.9