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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, 2016
Dec. 31, 2015
Dec. 31, 2014
Change in Projected Benefit Obligation [Roll Forward]      
Obligation at Jan. 1 $ 40,864 $ 44,342  
Service cost 775 954 $ 1,246
Interest cost 1,821 1,745 2,572
Medicare subsidy reimbursements 31 45  
Plan participants' contributions 653 687  
Actuarial (gain) loss 1,293 (3,793)  
Benefit payments (3,577) (3,116)  
Obligation at Dec. 31 41,860 40,864 44,342
Change in Fair Value of Plan Assets [Roll Forward]      
Fair value of plan assets at Jan. 1 42,684 45,356  
Actual return (loss) on plan assets 1,978 (421)  
Plan participants' contributions 653 687  
Employer contributions 559 178  
Benefit payments (3,577) (3,116)  
Fair value of plan assets at Dec. 31 42,297 42,684 45,356
Funded Status of Plans at Dec. 31 [Abstract]      
Funded status [1] 437 1,820  
Amounts Not Yet Recognized as Components of Net Periodic Benefit Cost (Credit) [Abstract]      
Net loss (12,595) (14,870)  
Prior service (credit) cost (2,630) (3,031)  
Total (15,225) (17,901)  
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 (1,004) (985)  
Noncurrent regulatory liabilities (14,221) (16,916)  
Total $ (15,225) $ (17,901)  
Significant Assumptions Used to Measure Benefit Obligations [Abstract]      
Discount rate for year-end valuation (as a percent) 4.13% 4.65%  
Mortality table RP 2014 RP 2014  
Health care costs trend rate - initial (as a percent) 5.50% 6.00%  
Ultimate health care trend assumption rate (as a percent) 4.50%    
Period until ultimate trend rate is reached (in years) 2 years    
Effect of One-Percentage Point Change in Assumed Health Care Cost Trend Rate [Abstract]      
One-percent increase in APBO $ 3,979    
One-percent decrease in APBO (3,389)    
One-percent increase in service and interest components 273    
One-percent decrease in service and interest components (231)    
Cash Flows [Abstract]      
Total contributions to Xcel Energy's postretirement health care plans during the year 600 $ 200 200
Expected contribution to postretirement health care plans during 2017 0    
Components of Net Periodic Benefit Cost (Credit) [Abstract]      
Service cost 775 954 1,246
Interest cost 1,821 1,745 2,572
Expected return on plan assets (2,377) (2,540) (3,247)
Amortization of prior service cost (credit) (401) (401) (401)
Amortization of net (gain) loss (583) (639) (321)
Net periodic pension cost $ (765) $ (881) $ (151)
Significant Assumptions Used to Measure Costs [Abstract]      
Discount rate (as a percent) 4.65% 4.08% 4.82%
Expected average long-term rate of return on assets (as a percent) 5.80% 5.80% 7.20%
Xcel Energy Inc.      
Cash Flows [Abstract]      
Total contributions to Xcel Energy's postretirement health care plans during the year $ 17,900 $ 18,300 $ 17,100
Expected contribution to postretirement health care plans during 2017 $ 11,800    
[1] Amounts are recognized in noncurrent assets on SPS’ balance sheet as of Dec. 31, 2016 and 2015