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Components of net periodic benefit cost (Details 3) - Qualified Plan - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2022
Dec. 31, 2021
Dec. 31, 2020
Underfunded Plan | Pension Benefits      
Components of net periodic benefit credit:      
Service cost $ 0 $ 0 $ 0
Interest cost 10,522 9,819 12,093
Expected return on assets (19,455) (19,576) (19,949)
Amortization of prior service credit 0 0 0
Recognized net actuarial loss (gain) 6,683 8,017 7,172
Net periodic benefit credit, including amount capitalized (2,250) (1,740) (684)
Less amount capitalized 0 0 0
Net periodic benefit cost (credit) (2,250) (1,740) (684)
Other changes in plan assets and benefit obligations recognized in accumulated comprehensive loss:      
Net (gain) loss 2,369 (265) 934
Amortization of actuarial loss (1,310) (1,286) (1,155)
Amortization of prior service credit 0 0 0
Reclassification of postretirement liability adjustment from regulatory asset 5,343 0 0
Total recognized in accumulated other comprehensive loss 6,402 (1,551) (221)
Other changes in plan assets and benefit obligations recognized in regulatory assets or liabilities:      
Net (gain) loss 9,757 (5,116) 4,546
Amortization of actuarial gain (loss) (5,373) (6,731) (6,017)
Amortization of prior service credit 0 0 0
Reclassification of postretirement liability adjustment from regulatory asset to OCI (5,343) 0 0
Total recognized in regulatory assets or liabilities (959) (11,847) (1,471)
Total recognized in net periodic benefit credit, accumulated other comprehensive loss and regulatory assets or liabilities 3,193 (15,138) (2,376)
Overfunded Plan | Other Postretirement Benefits      
Components of net periodic benefit credit:      
Service cost 1,416 1,600 1,532
Interest cost 1,896 1,862 2,437
Expected return on assets (5,288) (5,098) (5,019)
Amortization of prior service credit (1,398) (1,398) (1,398)
Recognized net actuarial loss (gain) (219) 24 287
Net periodic benefit credit, including amount capitalized (3,593) (3,010) (2,161)
Less amount capitalized 175 150 156
Net periodic benefit cost (credit) (3,768) (3,160) (2,317)
Other changes in plan assets and benefit obligations recognized in accumulated comprehensive loss:      
Net (gain) loss (4,141) (2,811) (259)
Amortization of actuarial loss (281) (135) (306)
Amortization of prior service credit 125 100 101
Reclassification of postretirement liability adjustment from regulatory asset (992) 0 0
Total recognized in accumulated other comprehensive loss (5,289) (2,846) (464)
Other changes in plan assets and benefit obligations recognized in regulatory assets or liabilities:      
Net (gain) loss 11,920 (6,292) (3,793)
Amortization of actuarial gain (loss) 500 110 19
Amortization of prior service credit 1,273 1,298 1,297
Reclassification of postretirement liability adjustment from regulatory asset to OCI 992 0 0
Total recognized in regulatory assets or liabilities 14,685 (4,884) (2,477)
Total recognized in net periodic benefit credit, accumulated other comprehensive loss and regulatory assets or liabilities $ 5,628 $ (10,890) $ (5,258)