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Benefit Plans and Other Postretirement Benefits, Pension Plan Benefit Obligations, Cash Flows and Benefit Costs (Details)
$ in Thousands
1 Months Ended 12 Months Ended
Jan. 31, 2016
USD ($)
Plan
Dec. 31, 2015
USD ($)
Plan
Dec. 31, 2014
USD ($)
Plan
Dec. 31, 2013
USD ($)
Plan
Significant Assumptions Used to Measure Costs [Abstract]        
Contributions to 401(k) and other defined contribution plans   $ 1,400 $ 1,400 $ 1,300
Pension Plans        
Defined Benefit Plan Disclosure [Line Items]        
Accumulated Benefit Obligation at Dec. 31   140,917 153,590  
Change in Projected Benefit Obligation [Roll Forward]        
Obligation at Jan. 1 $ 152,545 165,669 163,930  
Service cost   4,759 4,527 5,682
Interest cost   6,520 7,257 6,924
Actuarial (gain) loss   (11,159) 9,126  
Benefit payments   (13,244) (19,171)  
Obligation at Dec. 31   152,545 165,669 163,930
Change in Fair Value of Plan Assets [Roll Forward]        
Fair value of plan assets at Jan. 1 119,314 132,713 136,935  
Actual return (loss) on plan assets   (5,087) 6,916  
Employer contributions   4,932 8,033  
Benefit payments   (13,244) (19,171)  
Fair value of plan assets at Dec. 31   119,314 132,713 136,935
Funded Status of Plans at Dec. 31 [Abstract]        
Funded status [1]   (33,231) (32,956)  
Amounts Not Yet Recognized as Components of Net Periodic Benefit Cost [Abstract]        
Net loss   86,614 90,007  
Prior service (credit) cost   556 667  
Total   87,170 90,674  
Amounts Not Yet Recognized as Components of Net Periodic Benefit Cost Have Been Recorded as Follows Based Upon Expected Recovery in Rates [Abstract]        
Current regulatory assets   6,300 6,728  
Noncurrent regulatory assets   80,870 83,946  
Total   $ 87,170 $ 90,674  
Significant Assumptions Used to Measure Benefit Obligations [Abstract]        
Measurement date   December 31, 2015 December 31, 2014  
Discount rate for year-end valuation (as a percent)   4.66% 4.11%  
Expected average long-term increase in compensation level (as a percent)   4.00% 3.75%  
Mortality table   RP 2014 RP 2014  
Cash Flows [Abstract]        
Total contributions to Xcel Energy's pension plans during the period   $ 4,900 $ 8,000 11,300
Components of Net Periodic Benefit Cost (Credit) [Abstract]        
Service cost   4,759 4,527 5,682
Interest cost   6,520 7,257 6,924
Expected return on plan assets   (9,483) (9,642) (9,995)
Amortization of prior service cost (credit)   111 111 417
Amortization of net loss   6,804 6,617 7,924
Net periodic benefit cost   $ 8,711 $ 8,870 $ 10,952
Significant Assumptions Used to Measure Costs [Abstract]        
Discount rate (as a percent)   4.11% 4.75% 4.00%
Expected average long-term increase in compensation level (as a percent)   3.75% 3.75% 3.75%
Expected average long-term rate of return on assets (as a percent)   7.25% 7.25% 7.25%
Allocated costs for pension plans sponsored by Xcel Energy Inc.   $ 1,900 $ 1,700 $ 2,200
Expected average long-term rate of return on assets for next fiscal year (as a percent)   7.10%    
Number of years fair market value of plan assets is adjusted using calculated value method (in years)   5 years    
Annual adjustment rate used in calculated value method (as a percent)   20.00%    
Xcel Energy Inc. | Pension Plans        
Cash Flows [Abstract]        
Total contributions to Xcel Energy's pension plans during the period   $ 90,100 $ 130,600 $ 192,400
Number of pension plans to which contributions were made | Plan   4 4 4
Subsequent Event | Pension Plans        
Cash Flows [Abstract]        
Total contributions to Xcel Energy's pension plans during the period 7,400      
Subsequent Event | Xcel Energy Inc. | Pension Plans        
Cash Flows [Abstract]        
Total contributions to Xcel Energy's pension plans during the period $ 125,000      
Number of pension plans to which contributions were made | Plan 4      
[1] Amounts are recognized in noncurrent liabilities on NSP-Wisconsin’s consolidated balance sheets.