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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   $ 2,800 $ 2,600 $ 2,600
Pension Plans        
Defined Benefit Plan Disclosure [Line Items]        
Accumulated Benefit Obligation at Dec. 31   453,317 429,726  
Change in Projected Benefit Obligation [Roll Forward]        
Obligation at Jan. 1 $ 483,601 467,394 500,690  
Service cost   9,761 11,006 9,184
Interest cost   21,259 20,184 20,444
Actuarial (gain) loss   25,053 (35,154)  
Transfer (to) from other plan   (3,305) (2,843)  
Benefit payments   (36,561) (26,489)  
Obligation at Dec. 31   483,601 467,394 500,690
Change in Fair Value of Plan Assets [Roll Forward]        
Fair value of plan assets at Jan. 1 380,395 378,913 402,269  
Actual return (loss) on plan assets   23,306 (6,013)  
Employer contributions   18,088 11,651  
Transfer (to) from other plan   (3,351) (2,505)  
Benefit payments   (36,561) (26,489)  
Fair value of plan assets at Dec. 31   380,395 378,913 402,269
Funded Status of Plans at Dec. 31 [Abstract]        
Funded status [1]   (103,206) (88,481)  
Amounts Not Yet Recognized as Components of Net Periodic Benefit Cost [Abstract]        
Net loss   247,381 236,107  
Total   247,381 236,107  
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   13,524 13,690  
Noncurrent regulatory assets   233,857 222,417  
Total   $ 247,381 $ 236,107  
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   $ 18,100 $ 11,700 4,900
Components of Net Periodic Benefit Cost (Credit) [Abstract]        
Service cost   9,761 11,006 9,184
Interest cost   21,259 20,184 20,444
Expected return on plan assets   (27,602) (28,610) (26,179)
Amortization of prior service cost (credit)   0 39 54
Amortization of net (gain) loss   11,986 15,087 13,326
Net periodic pension cost   15,404 17,706 16,829
Defined Benefit Plan Credits (Costs) Not Recognized Due To Effects of Regulation   (2,042) (2,597) (3,170)
Net benefit cost recognized for financial reporting   $ 17,446 $ 20,303 $ 19,999
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)   6.78% 7.22% 6.90%
Allocated costs for pension plans sponsored by Xcel Energy Services Inc.   $ 4,400 $ 4,800 $ 4,100
Expected average long-term rate of return on assets for next fiscal year (as a percent)   6.80%    
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 23,000      
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 SPS’ balance sheets.