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Commitments and Contingencies, Fuel Contracts (Details) (USD $)
In Millions, unless otherwise specified
12 Months Ended
Dec. 31, 2012
Fuel Contracts [Abstract]  
Minimum annual tolerance band percentage for future rate recovery or refund of fuel costs (in hundredths) 2.00%
Coal [Member]
 
Fuel Contracts [Abstract]  
2013 6.3
2014 1.2
2015 0.7
2016 0.7
2017 0.7
Thereafter 4.6
Total 14.2 [1]
Natural Gas Supply [Member]
 
Fuel Contracts [Abstract]  
2013 10.3
2014 0.3
2015 0.3
2016 0.3
2017 0.3
Thereafter 0
Total 11.5 [1]
Natural Gas Storage and Transportation [Member]
 
Fuel Contracts [Abstract]  
2013 13.2
2014 11.3
2015 10.7
2016 10.7
2017 8.6
Thereafter 20.5
Total 75.0 [1]
[1] Excludes additional amounts allocated to NSP-Wisconsin through intercompany charges.