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Change in benefit obligations and plan assets (Details) - Qualified Plan - USD ($) $ in Thousands | 12 Months Ended
--- | ---
Dec. 31, 2020 | Dec. 31, 2019 | Dec. 31, 2018
Underfunded Plan | Pension Benefits |  |  |
Change in benefit obligation: |  |  |
Benefit obligation at beginning of year | $ 421,166 | $ 391,602 |
Service cost | 0 | 0 | $ 0
Interest cost | 12,093 | 15,225 | 14,591
Plan participants' contributions | 0 | 0 |
Actuarial loss | 27,737 | 40,219 |
Benefits paid | (23,636) | (25,880) |
Benefit obligation at end of year | 437,360 | 421,166 | 391,602
Change in net plan assets: |  |  |
Fair value of plan assets at beginning of year | 365,264 | 307,809 |
Actual return on plan assets | 42,206 | 58,409 |
Employer contribution | 0 | 24,926 |
Plan participants' contributions | 0 | 0 |
Benefits paid | (23,636) | (25,880) |
Fair value of net plan assets at end of year | 383,834 | 365,264 | 307,809
Funded status - over (under) | (53,526) | (55,902) |
Amounts recognized in the Consolidated Balance Sheets at December 31: |  |  |
Noncurrent assets - other | 0 | 0 |
Other accrued liabilities | 0 | 0 |
Noncurrent liabilities - other | 53,526 | 55,902 |
Benefit obligation assets (liabilities) - net amount recognized | (53,526) | (55,902) |
Amounts recognized in accumulated other comprehensive loss: |  |  |
Actuarial loss | 27,527 | 27,748 |
Prior service credit | 0 | 0 |
Total | 27,527 | 27,748 |
Amounts recognized in regulatory assets or liabilities: |  |  |
Actuarial (gain) loss | 154,013 | 155,484 |
Prior service credit | 0 | 0 |
Total | 154,013 | 155,484 |
Overfunded Plan | Other Postretirement Benefits |  |  |
Change in benefit obligation: |  |  |
Benefit obligation at beginning of year | 83,614 | 81,201 |
Service cost | 1,532 | 1,142 | 1,494
Interest cost | 2,437 | 2,986 | 2,899
Plan participants' contributions | 752 | 1,040 |
Actuarial loss | 2,203 | 2,632 |
Benefits paid | (4,383) | (5,387) |
Benefit obligation at end of year | 86,155 | 83,614 | 81,201
Change in net plan assets: |  |  |
Fair value of plan assets at beginning of year | 94,587 | 82,516 |
Actual return on plan assets | 10,249 | 15,731 |
Employer contribution | 434 | 687 |
Plan participants' contributions | 752 | 1,040 |
Benefits paid | (4,383) | (5,387) |
Fair value of net plan assets at end of year | 101,639 | 94,587 | $ 82,516
Funded status - over (under) | 15,484 | 10,973 |
Amounts recognized in the Consolidated Balance Sheets at December 31: |  |  |
Noncurrent assets - other | 36,769 | 30,475 |
Other accrued liabilities | 622 | 647 |
Noncurrent liabilities - other | 20,663 | 18,855 |
Benefit obligation assets (liabilities) - net amount recognized | 15,484 | 10,973 |
Amounts recognized in accumulated other comprehensive loss: |  |  |
Actuarial loss | 5,557 | 6,118 |
Prior service credit | (634) | (731) |
Total | 4,923 | 5,387 |
Amounts recognized in regulatory assets or liabilities: |  |  |
Actuarial (gain) loss | (8,228) | (4,450) |
Prior service credit | (6,808) | (8,109) |
Total | $ (15,036) | $ (12,559) |

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