Regulatory Assets and Liabilities (Details) (USD $) In Millions, unless otherwise specified	Dec. 31, 2013	Dec. 31, 2012
Regulatory Assets [Line Items]
Regulatory assets earnings return based on short-term rates	$ 82.7
Remaining Amounts of Regulatory Assets for which No Return on Investment During Recovery Period is Provided	10.1
Regulatory Assets
Regulatory Assets	1,108.5	1,380.3
Deferred unrecognized pension costs [Member]
Regulatory Assets
Regulatory Assets	537.6	731.5
Deferred income tax related [Member]
Regulatory Assets
Regulatory Assets	169.5	176.5
Escrowed electric transmission costs [Member]
Regulatory Assets
Regulatory Assets	126.8	114.1
Escrowed conservation [Member]
Regulatory Assets
Regulatory Assets	66.9	73.5
Deferred plant related -- capital lease [Member]
Regulatory Assets
Regulatory Assets	56.5	66.6
Deferred environmental costs [Member]
Regulatory Assets
Regulatory Assets	47.0	47.4
Other, net [Member]
Regulatory Assets
Regulatory Assets	$ 104.2	$ 170.7