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Commitments and Contingencies, Fuel Contracts (Details) (USD $)
In Millions, unless otherwise specified
12 Months Ended
Dec. 31, 2013
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]  
2014 $ 6.2
2015 1.3
2016 0.7
2017 0.7
2018 0.7
Thereafter 4.1
Total 13.7 [1]
Natural Gas Supply [Member]
 
Fuel Contracts [Abstract]  
2014 13.4
2015 0.3
2016 0.3
2017 0.2
2018 0
Thereafter 0
Total 14.2 [1]
Natural Gas Storage and Transportation [Member]
 
Fuel Contracts [Abstract]  
2014 11.9
2015 11.6
2016 11.7
2017 9.5
2018 4.4
Thereafter 18.9
Total $ 68.0 [1]
[1] Excludes additional amounts allocated to NSP-Wisconsin through intercompany charges.