Regulatory Assets and Liabilities (Details) (USD $) In Millions, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011
Regulatory Assets [Line Items]
Remaining Amounts of Regulatory Assets for which No Return on Investment During Recovery Period is Provided	$ 6.6	$ 11.0
Regulatory Assets
Regulatory Assets	1,380.3	1,265.7
Deferred unrecognized pension costs [Member]
Regulatory Assets
Regulatory Assets	731.5	647.8
Deferred income tax related [Member]
Regulatory Assets
Regulatory Assets	176.5	121.2
Escrowed electric transmission costs [Member]
Regulatory Assets
Regulatory Assets	114.1	118.3
Escrowed conservation [Member]
Regulatory Assets
Regulatory Assets	73.5	31.5
Deferred unrecognized OPEB costs [Member]
Regulatory Assets
Regulatory Assets	61.6	102.9
Deferred plant related -- capital lease [Member]
Regulatory Assets
Regulatory Assets	66.6	73.2
Deferred environmental costs [Member]
Regulatory Assets
Regulatory Assets	47.4	48.5
Other, net [Member]
Regulatory Assets
Regulatory Assets	$ 109.1	$ 122.3