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Components of net periodic benefit cost (Details 3) - Qualified Plan - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2021
Dec. 31, 2020
Dec. 31, 2019
Underfunded Plan | Pension Benefits      
Components of net periodic benefit cost (credit):      
Service cost $ 0 $ 0 $ 0
Interest cost 9,819 12,093 15,225
Expected return on assets (19,576) (19,949) (18,236)
Amortization of prior service credit 0 0 0
Recognized net actuarial loss 8,017 7,172 5,548
Net periodic benefit cost (credit), including amount capitalized (1,740) (684) 2,537
Less amount capitalized 0 0 0
Net periodic benefit cost (credit) (1,740) (684) 2,537
Other changes in plan assets and benefit obligations recognized in accumulated comprehensive loss:      
Net (gain) loss (265) 934 (144)
Amortization of actuarial loss (1,286) (1,155) (904)
Amortization of prior service credit 0 0 0
Total recognized in accumulated other comprehensive loss (1,551) (221) (1,048)
Other changes in plan assets and benefit obligations recognized in regulatory assets or liabilities:      
Net (gain) loss (5,116) 4,546 189
Amortization of actuarial gain (loss) (6,731) (6,017) (4,644)
Amortization of prior service credit 0 0 0
Total recognized in regulatory assets or liabilities (11,847) (1,471) (4,455)
Total recognized in net periodic benefit cost (credit), accumulated other comprehensive loss and regulatory assets or liabilities (15,138) (2,376) (2,966)
Overfunded Plan | Other Postretirement Benefits      
Components of net periodic benefit cost (credit):      
Service cost 1,600 1,532 1,142
Interest cost 1,862 2,437 2,986
Expected return on assets (5,098) (5,019) (4,804)
Amortization of prior service credit (1,398) (1,398) (1,398)
Recognized net actuarial loss 24 287 353
Net periodic benefit cost (credit), including amount capitalized (3,010) (2,161) (1,721)
Less amount capitalized 150 156 113
Net periodic benefit cost (credit) (3,160) (2,317) (1,834)
Other changes in plan assets and benefit obligations recognized in accumulated comprehensive loss:      
Net (gain) loss (2,811) (259) (127)
Amortization of actuarial loss (135) (306) (110)
Amortization of prior service credit 100 101 100
Total recognized in accumulated other comprehensive loss (2,846) (464) (137)
Other changes in plan assets and benefit obligations recognized in regulatory assets or liabilities:      
Net (gain) loss (6,292) (3,793) (8,168)
Amortization of actuarial gain (loss) 110 19 (242)
Amortization of prior service credit 1,298 1,297 1,297
Total recognized in regulatory assets or liabilities (4,884) (2,477) (7,113)
Total recognized in net periodic benefit cost (credit), accumulated other comprehensive loss and regulatory assets or liabilities $ (10,890) $ (5,258) $ (9,084)