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Assumptions (Details) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Minimum      
Defined Benefit Plan Disclosure [Line Items]      
Trend Year 2027 2027  
Maximum      
Defined Benefit Plan Disclosure [Line Items]      
Trend Year 2029 2028  
Black Hills Corporation | Minimum      
Defined Benefit Plan Disclosure [Line Items]      
Post-65 for next year - All Plans 6.10% 6.40%  
Black Hills Corporation | Maximum      
Defined Benefit Plan Disclosure [Line Items]      
Post-65 for next year - All Plans 4.92% 4.92%  
Black Hills Service Company | Minimum      
Defined Benefit Plan Disclosure [Line Items]      
Post-65 Ultimate trend rate - Black Hills Corp 4.50% 4.50%  
Black Hills Service Company | Maximum      
Defined Benefit Plan Disclosure [Line Items]      
Post-65 Ultimate trend rate - Black Hills Corp 4.50% 4.50%  
Pension Plan      
Defined Benefit Plan Disclosure [Line Items]      
Discount rate 2.56% 3.27% 4.40%
Rate of increase in compensation levels 3.34% 3.49% 3.52%
Expected long-term rate of return on assets 5.25% 6.00% 6.25%
Rate of increase in compensation levels 3.49% 3.52% 3.43%
Defined Benefit Plan Assumptions Used In Calculating Net Periodic Benefit Cost Expected Rate of Return On Assets For Next Fiscal Year 4.50%    
2021 $ 25,842    
2022 26,658    
2023 27,581    
2024 28,284    
2025 29,062    
2026-2030 $ 144,273    
Pension Plan | Black Hills Corporation      
Defined Benefit Plan Disclosure [Line Items]      
Discount Rate 3.27% 4.40% 3.71%
Pension Plan | Black Hills Corporation Pension Plan      
Defined Benefit Plan Disclosure [Line Items]      
Post-65 for next year - All Plans 2.56%    
Supplemental Employee Retirement Plan      
Defined Benefit Plan Disclosure [Line Items]      
Discount rate 2.41% 3.14% 4.34%
Rate of increase in compensation levels 5.00% 5.00% 5.00%
Rate of increase in compensation levels 5.00% 5.00% 5.00%
2021 $ 1,927    
2022 1,968    
2023 2,033    
2024 2,231    
2025 2,690    
2026-2030 $ 13,117    
Supplemental Employee Retirement Plan | Black Hills Corporation      
Defined Benefit Plan Disclosure [Line Items]      
Discount Rate 3.14% 4.34% 3.67%
Other Postretirement Benefits Plan      
Defined Benefit Plan Disclosure [Line Items]      
Discount rate 2.41% 3.15% 4.28%
Expected long-term rate of return on assets 2.35% 3.00% 3.93%
2021 $ 6,108    
2022 5,965    
2023 5,725    
2024 5,532    
2025 5,244    
2026-2030 $ 22,872    
Other Postretirement Benefits Plan | Black Hills Corporation      
Defined Benefit Plan Disclosure [Line Items]      
Discount Rate 3.15% 4.28% 3.60%