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Commitments and Contingencies, Environmental Contingencies - Site Contingencies (Details)
$ in Thousands
1 Months Ended 12 Months Ended
Dec. 31, 2015
USD ($)
Site
Dec. 31, 2015
USD ($)
Site
Dec. 31, 2014
USD ($)
Manufactured Gas Plant (MGP) Site [Abstract]      
Liability for estimated cost of remediating sites, current $ 17,155 $ 17,155 $ 29,116
Ashland MGP Site      
Manufactured Gas Plant (MGP) Site [Abstract]      
Number of properties included in superfund site which NSP-Wisconsin does not own | Site 2 2  
Liability for estimated cost of remediating sites, current $ 17,000 $ 17,000 28,900
Accrual for Environmental Loss Contingencies, Gross 94,400 $ 94,400 107,600
Public Utilities, Annual recovery collected through base rates     4,700
Ashland MGP Site - Phase I Project Area      
Manufactured Gas Plant (MGP) Site [Abstract]      
Approved amortization period for recovery of remediation costs in natural gas rates (in years)   10 years  
Accrual for Environmental Loss Contingencies, Gross 65,000 $ 65,000  
Estimated amount spent on cleanup 47,000 $ 47,000  
Carrying cost percentage to be applied to unamortized regulatory asset   3.00%  
Ashland MGP Site - Sediments      
Manufactured Gas Plant (MGP) Site [Abstract]      
Estimated cost of remediating site, low end of range 63,000 $ 63,000  
Estimated cost of remediating site, high end of range $ 77,000 $ 77,000  
Potential percent of increase to the high end of the range of estimated site remediation costs (in hundredths) 50.00% 50.00%  
Potential percent of decrease to the low end of the range of estimated site remediation costs (in hundredths) 30.00% 30.00%  
Other MGP Sites      
Manufactured Gas Plant (MGP) Site [Abstract]      
Liability for estimated cost of remediating sites $ 200 $ 200 $ 200
Number of identified MGP sites under current investigation and/or remediation | Site 1 1  
PSCW Proceeding - Electric and Gas Rate Case 2016 - Gas Rates 2016 | Ashland MGP Site      
Manufactured Gas Plant (MGP) Site [Abstract]      
Public Utilities, Approved annual recovery collected through base rates $ 7,600    
Other PRPs | NSP-Wisconsin | Ashland MGP Site      
Manufactured Gas Plant (MGP) Site [Abstract]      
Number of PRPs that have contributed to remediation site   3  
Contributions to site cleanup by PRPs   $ 15,900