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Benefit Plans and Other Postretirement Benefits, Postretirement Benefit Plan Benefit Obligations, Cash Flows and Benefit Costs (Details) - Postretirement Benefit Plan - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2015
Dec. 31, 2014
Dec. 31, 2013
Change in Projected Benefit Obligation [Roll Forward]      
Obligation at Jan. 1 $ 44,342 $ 54,982  
Service cost 954 1,246 $ 1,368
Interest cost 1,745 2,572 2,352
Medicare subsidy reimbursements 45 18  
Plan participants' contributions 687 728  
Actuarial (gain) loss (3,793) (11,828)  
Benefit payments (3,116) (3,376)  
Obligation at Dec. 31 40,864 44,342 54,982
Change in Fair Value of Plan Assets [Roll Forward]      
Fair value of plan assets at Jan. 1 45,356 46,738  
Actual return (loss) on plan assets (421) 1,073  
Plan participants' contributions 687 728  
Employer contributions 178 193  
Benefit payments (3,116) (3,376)  
Fair value of plan assets at Dec. 31 42,684 45,356 46,738
Funded Status of Plans at Dec. 31 [Abstract]      
Funded status [1] 1,820 1,014  
Amounts Not Yet Recognized as Components of Net Periodic Benefit Cost (Credit) [Abstract]      
Net loss (14,870) (14,677)  
Prior service (credit) cost (3,031) (3,432)  
Total (17,901) (18,109)  
Amounts Not Yet Recognized as Components of Net Periodic Benefit Cost (Credit) Have Been Recorded as Follows Based Upon Expected Recovery in Rates [Abstract]      
Current regulatory liabilities (985) (892)  
Noncurrent regulatory liabilities (16,916) (17,217)  
Total $ (17,901) $ (18,109)  
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) 4.65% 4.08%  
Mortality table RP 2014 RP 2014  
Health care costs trend rate - initial (as a percent) 6.00% 6.50%  
Ultimate health care trend assumption rate (as a percent) 4.50%    
Period until ultimate trend rate is reached (in years) 3 years    
Effect of One-Percentage Point Change in Assumed Health Care Cost Trend Rate [Abstract]      
One-percent increase in APBO $ 3,944    
One-percent decrease in APBO (3,355)    
One-percent increase in service and interest components 307    
One-percent decrease in service and interest components (255)    
Cash Flows [Abstract]      
Total contributions to Xcel Energy's postretirement health care plans during the year 200 $ 200 100
Expected contribution to postretirement health care plans during 2015 0    
Components of Net Periodic Benefit Cost (Credit) [Abstract]      
Service cost 954 1,246 1,368
Interest cost 1,745 2,572 2,352
Expected return on plan assets (2,540) (3,247) (3,183)
Amortization of prior service cost (credit) (401) (401) (484)
Amortization of net (gain) loss (639) (321) (6)
Net periodic benefit cost (credit) $ (881) $ (151) $ 47
Significant Assumptions Used to Measure Costs [Abstract]      
Discount rate (as a percent) 4.08% 4.82% 4.10%
Expected average long-term rate of return on assets (as a percent) 5.80% 7.20% 7.11%
Xcel Energy Inc.      
Cash Flows [Abstract]      
Total contributions to Xcel Energy's postretirement health care plans during the year $ 18,300 $ 17,100 $ 17,600
Expected contribution to postretirement health care plans during 2015 $ 12,300    
[1] Amounts are recognized in noncurrent assets and noncurrent liabilities on SPS’ balance sheet as of Dec. 31, 2015 and 2014, respectively.