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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, 2017
USD ($)
Dec. 31, 2016
USD ($)
Plan
Dec. 31, 2015
USD ($)
Plan
Dec. 31, 2014
USD ($)
Plan
Significant Assumptions Used to Measure Costs [Abstract]        
Contributions to 401(k) and other defined contribution plans   $ 1,400 $ 1,400 $ 1,400
Pension Plans        
Defined Benefit Plan Disclosure [Line Items]        
Accumulated Benefit Obligation at Dec. 31   146,448 140,917  
Change in Projected Benefit Obligation [Roll Forward]        
Obligation at Jan. 1 $ 157,457 152,545 165,669  
Service cost   4,417 4,759 4,527
Interest cost   6,816 6,520 7,257
Plan amendments   305 0  
Actuarial (gain) loss   7,315 (11,159)  
Benefit payments   (13,941) (13,244)  
Obligation at Dec. 31   157,457 152,545 165,669
Change in Fair Value of Plan Assets [Roll Forward]        
Fair value of plan assets at Jan. 1 118,976 119,314 132,713  
Actual return (loss) on plan assets   6,163 (5,087)  
Employer contributions   7,440 4,932  
Benefit payments   (13,941) (13,244)  
Fair value of plan assets at Dec. 31   118,976 119,314 132,713
Funded Status of Plans at Dec. 31 [Abstract]        
Funded status [1]   (38,481) (33,231)  
Amounts Not Yet Recognized as Components of Net Periodic Benefit Cost [Abstract]        
Net loss   91,531 86,614  
Prior service (credit) cost   750 556  
Total   92,281 87,170  
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   5,972 6,300  
Noncurrent regulatory assets   86,309 80,870  
Total   $ 92,281 $ 87,170  
Significant Assumptions Used to Measure Benefit Obligations [Abstract]        
Discount rate for year-end valuation (as a percent)   4.13% 4.66%  
Expected average long-term increase in compensation level (as a percent)   3.75% 4.00%  
Mortality table   RP 2014 RP 2014  
Cash Flows [Abstract]        
Total contributions to Xcel Energy's pension plans during the period   $ 7,400 $ 4,900 8,000
Components of Net Periodic Benefit Cost (Credit) [Abstract]        
Service cost   4,417 4,759 4,527
Interest cost   6,816 6,520 7,257
Expected return on plan assets   (9,157) (9,483) (9,642)
Amortization of prior service cost (credit)   111 111 111
Amortization of net loss   5,392 6,804 6,617
Net periodic benefit cost   $ 7,579 $ 8,711 $ 8,870
Significant Assumptions Used to Measure Costs [Abstract]        
Discount rate (as a percent)   4.66% 4.11% 4.75%
Expected average long-term increase in compensation level (as a percent)   4.00% 3.75% 3.75%
Expected average long-term rate of return on assets (as a percent)   7.10% 7.25% 7.25%
Allocated costs for pension plans sponsored by Xcel Energy Inc.   $ 1,600 $ 1,900 $ 1,700
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   $ 125,200 $ 90,100 $ 130,600
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 9,000      
Subsequent Event | Xcel Energy Inc. | Pension Plans        
Cash Flows [Abstract]        
Total contributions to Xcel Energy's pension plans during the period $ 150,000      
[1] Amounts are recognized in noncurrent liabilities on NSP-Wisconsin’s consolidated balance sheets.