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Change in benefit obligations and plan assets (Details) - Qualified Plan - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2021
Dec. 31, 2020
Dec. 31, 2019
Underfunded Plan | Pension Benefits      
Change in benefit obligation:      
Benefit obligation at beginning of year $ 437,360 $ 421,166  
Service cost 0 0 $ 0
Interest cost 9,819 12,093 15,225
Plan participants' contributions 0 0  
Actuarial (gain) loss (12,140) 27,737  
Benefits paid (23,542) (23,636)  
Benefit obligation at end of year 411,497 437,360 421,166
Change in net plan assets:      
Fair value of plan assets at beginning of year 383,834 365,264  
Actual return on plan assets 12,817 42,206  
Employer contribution 0 0  
Plan participants' contributions 0 0  
Benefits paid (23,542) (23,636)  
Fair value of net plan assets at end of year 373,109 383,834 365,264
Funded status - over (under) (38,388) (53,526)  
Amounts recognized in the Consolidated Balance Sheets at December 31:      
Noncurrent assets - other 0 0  
Other accrued liabilities 0 0  
Noncurrent liabilities - other 38,388 53,526  
Benefit obligation assets (liabilities) - net amount recognized (38,388) (53,526)  
Amounts recognized in accumulated other comprehensive loss:      
Actuarial loss 25,976 27,527  
Prior service credit 0 0  
Total 25,976 27,527  
Amounts recognized in regulatory assets or liabilities:      
Actuarial (gain) loss 142,166 154,013  
Prior service credit 0 0  
Total 142,166 154,013  
Overfunded Plan | Other Postretirement Benefits      
Change in benefit obligation:      
Benefit obligation at beginning of year 86,155 83,614  
Service cost 1,600 1,532 1,142
Interest cost 1,862 2,437 2,986
Plan participants' contributions 641 752  
Actuarial (gain) loss (12,802) 2,203  
Benefits paid (3,996) (4,383)  
Benefit obligation at end of year 73,460 86,155 83,614
Change in net plan assets:      
Fair value of plan assets at beginning of year 101,639 94,587  
Actual return on plan assets 1,398 10,249  
Employer contribution 476 434  
Plan participants' contributions 641 752  
Benefits paid (3,996) (4,383)  
Fair value of net plan assets at end of year 100,158 101,639 $ 94,587
Funded status - over (under) 26,698 15,484  
Amounts recognized in the Consolidated Balance Sheets at December 31:      
Noncurrent assets - other 45,863 36,769  
Other accrued liabilities 544 622  
Noncurrent liabilities - other 18,621 20,663  
Benefit obligation assets (liabilities) - net amount recognized 26,698 15,484  
Amounts recognized in accumulated other comprehensive loss:      
Actuarial loss 2,367 5,557  
Prior service credit (290) (634)  
Total 2,077 4,923  
Amounts recognized in regulatory assets or liabilities:      
Actuarial (gain) loss (14,727) (8,228)  
Prior service credit (5,193) (6,808)  
Total $ (19,920) $ (15,036)