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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   $ 2,600 $ 2,600 $ 2,400
Pension Plans        
Defined Benefit Plan Disclosure [Line Items]        
Accumulated Benefit Obligation at Dec. 31   429,726 458,793  
Change in Projected Benefit Obligation [Roll Forward]        
Obligation at Jan. 1 $ 467,394 500,690 434,307  
Service cost   11,006 9,184 9,615
Interest cost   20,184 20,444 17,908
Actuarial (gain) loss   (35,154) 63,209  
Transfer (to) from other plan   (2,843) (1,939)  
Benefit payments   (26,489) (24,515)  
Obligation at Dec. 31   467,394 500,690 434,307
Change in Fair Value of Plan Assets [Roll Forward]        
Fair value of plan assets at Jan. 1 378,913 402,269 393,393  
Actual return (loss) on plan assets   (6,013) 30,159  
Employer contributions   11,651 4,869  
Transfer (to) from other plan   (2,505) (1,637)  
Benefit payments   (26,489) (24,515)  
Fair value of plan assets at Dec. 31   378,913 402,269 393,393
Funded Status of Plans at Dec. 31 [Abstract]        
Funded status [1]   (88,481) (98,421)  
Amounts Not Yet Recognized as Components of Net Periodic Benefit Cost [Abstract]        
Net loss   236,107 252,063  
Prior service (credit) cost   0 39  
Total   236,107 252,102  
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   13,690 14,437  
Noncurrent regulatory assets   222,417 237,665  
Total   $ 236,107 $ 252,102  
Significant Assumptions Used to Measure Benefit Obligations [Abstract]        
Measurement date   12/31/2015 12/31/2014  
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   $ 11,700 $ 4,900 22,000
Components of Net Periodic Benefit Cost (Credit) [Abstract]        
Service cost   11,006 9,184 9,615
Interest cost   20,184 20,444 17,908
Expected return on plan assets   (28,610) (26,179) (23,970)
Amortization of prior service cost (credit)   39 54 870
Amortization of net (gain) loss   15,087 13,326 17,148
Net periodic benefit cost (credit)   17,706 16,829 21,571
(Costs) credits not recognized due to effects of regulation   2,597 3,170 (1,269)
Net benefit cost recognized for financial reporting   $ 20,303 $ 19,999 $ 20,302
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)   7.22% 6.90% 6.49%
Allocated costs for pension plans sponsored by Xcel Energy Services Inc.   $ 4,800 $ 4,100 $ 4,900
Expected average long-term rate of return on assets for next fiscal year (as a percent)   6.78%    
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 18,000      
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 SPS’ balance sheets.