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Benefit Plans and Other Postretirement Benefits, Postretirement Benefit Plan Benefit Obligations, Cash Flows and Benefit Costs (Details) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Funded Status of Plans at Dec. 31 [Abstract]      
Noncurrent liabilities $ (50,027) $ (55,164)  
Significant Assumptions Used to Measure Costs [Abstract]      
Expected average long-term rate of return on assets (as a percent) 7.10% 7.10%  
Other Postretirement Benefits Plan [Member]      
Change in Projected Benefit Obligation [Roll Forward]      
Obligation at Jan. 1 $ 14,973 $ 14,718  
Service cost 29 24 $ 29
Interest cost 590 651 653
Medical subsidy reimbursements 0 7  
Plan participants' contributions 71 87  
Actuarial (gain) loss 2,069 775  
Benefit payments (1,368) (1,289)  
Obligation at Dec. 31 16,364 14,973 14,718
Change in Fair Value of Plan Assets [Roll Forward]      
Fair value of plan assets at Jan. 1 543 418  
Actual return (loss) on plan assets (6) (12)  
Plan participants' contributions 71 87  
Employer contributions 1,838 1,339  
Benefit payments (1,368) (1,289)  
Fair value of plan assets at Dec. 31 1,078 543 418
Funded Status of Plans at Dec. 31 [Abstract]      
Funded status (15,286) (14,430)  
Current liabilities (269) (822)  
Noncurrent liabilities (15,017) (13,608)  
Net postretirement amounts recognized on consolidated balance sheets (15,286) (14,430)  
Amount Not Yet Recognized as Components of Net Periodic Benefit Cost Have Been Recorded as Follows Based Upon Expected Recovery in Rates [Abstract]      
Net loss 10,553 8,883  
Prior service (credit) cost (1,783) (2,134)  
Total 8,770 6,749  
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 110 0  
Noncurrent regulatory assets 8,660 6,749  
Total $ 8,770 $ 6,749  
Significant Assumptions Used to Measure Benefit Obligations [Abstract]      
Discount rate for year-end valuation (as a percent) 3.62% 4.13%  
Mortality table RP 2014 RP 2014  
Defined Benefit Plan, Health Care Cost Trend Rate Assumed, Pre-65 7.00% 5.50%  
Defined Benefit Plan, Health Care Cost Trend Rate Assumed, Post-65 5.50% 5.50%  
Ultimate health care trend assumption rate (as a percent) 4.50%    
Period until ultimate trend rate is reached (in years) 5 years    
Effect of One-Percentage Point Change in Assumed Health Care Cost Trend Rate [Abstract]      
One-percent increase in APBO $ 1,588    
One-percent decrease in APBO (1,344)    
One-percent increase in service and interest components 65    
One-percent decrease in service and interest components (55)    
Cash Flows [Abstract]      
Total contributions to Xcel Energy's postretirement health care plans during the year 2,000 $ 1,000 1,000
Expected contribution to postretirement health care plans during 2018 1,000    
Components of Net Periodic Benefit Cost (Credit) [Abstract]      
Service cost 29 24 29
Interest cost 590 651 653
Expected return on plan assets (31) (24) (30)
Amortization of prior service cost (credit) (351) (351) (351)
Amortization of net loss 436 330 456
Net benefit cost recognized for financial reporting $ 673 $ 630 $ 757
Significant Assumptions Used to Measure Costs [Abstract]      
Discount rate (as a percent) 4.13% 4.65% 4.08%
Expected average long-term rate of return on assets (as a percent) 5.80% 5.80% 5.80%
Xcel Energy Inc. | Other Postretirement Benefits Plan [Member]      
Cash Flows [Abstract]      
Total contributions to Xcel Energy's postretirement health care plans during the year $ 20,000 $ 18,000 $ 18,000
Expected contribution to postretirement health care plans during 2018 $ 12,000