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Change in benefit obligations and plan assets (Details) - Qualified Plan - USD ($) $ in Thousands | 12 Months Ended
--- | ---
Dec. 31, 2019 | Dec. 31, 2018 | Dec. 31, 2017
Underfunded Plan | Pension Benefits |  |  |
Change in benefit obligation: |  |  |
Benefit obligation at beginning of year | $ 391,602 | $ 445,923 |
Service cost | 0 | 0 | $ 0
Interest cost | 15,225 | 14,591 | 16,207
Plan participants' contributions | 0 | 0 |
Actuarial (gain) loss | 40,219 | (32,637) |
Benefits paid | (25,880) | (36,275) |
Benefit obligation at end of year | 421,166 | 391,602 | 445,923
Change in net plan assets: |  |  |
Fair value of plan assets at beginning of year | 307,809 | 354,384 |
Actual gain (loss) on plan assets | 58,409 | (21,138) |
Employer contribution | 24,926 | 10,838 |
Plan participants' contributions | 0 | 0 |
Benefits paid | (25,880) | (36,275) |
Fair value of net plan assets at end of year | 365,264 | 307,809 | 354,384
Funded status - over (under) | (55,902) | (83,793) |
Amounts recognized in the Consolidated Balance Sheets at December 31: |  |  |
Deferred charges and other assets - other | 0 | 0 |
Other accrued liabilities | 0 | 0 |
Deferred credits and other liabilities - other | 55,902 | 83,793 |
Benefit obligation assets (liabilities) - net amount recognized | (55,902) | (83,793) |
Amounts recognized in accumulated other comprehensive loss: |  |  |
Actuarial loss | 27,748 | 28,796 |
Prior service credit | 0 | 0 |
Total | 27,748 | 28,796 |
Amounts recognized in regulatory assets or liabilities: |  |  |
Actuarial (gain) loss | 155,484 | 159,939 |
Prior service credit | 0 | 0 |
Total | 155,484 | 159,939 |
Overfunded Plan | Other Postretirement Benefits |  |  |
Change in benefit obligation: |  |  |
Benefit obligation at beginning of year | 81,201 | 91,206 |
Service cost | 1,142 | 1,494 | 1,508
Interest cost | 2,986 | 2,899 | 3,265
Plan participants' contributions | 1,040 | 1,282 |
Actuarial (gain) loss | 2,632 | (10,115) |
Benefits paid | (5,387) | (5,565) |
Benefit obligation at end of year | 83,614 | 81,201 | 91,206
Change in net plan assets: |  |  |
Fair value of plan assets at beginning of year | 82,516 | 88,739 |
Actual gain (loss) on plan assets | 15,731 | (2,781) |
Employer contribution | 687 | 842 |
Plan participants' contributions | 1,040 | 1,281 |
Benefits paid | (5,387) | (5,565) |
Fair value of net plan assets at end of year | 94,587 | 82,516 | $ 88,739
Funded status - over (under) | 10,973 | 1,315 |
Amounts recognized in the Consolidated Balance Sheets at December 31: |  |  |
Deferred charges and other assets - other | 30,475 | 20,843 |
Other accrued liabilities | 647 | 660 |
Deferred credits and other liabilities - other | 18,855 | 18,868 |
Benefit obligation assets (liabilities) - net amount recognized | 10,973 | 1,315 |
Amounts recognized in accumulated other comprehensive loss: |  |  |
Actuarial loss | 6,118 | 6,372 |
Prior service credit | (731) | (848) |
Total | 5,387 | 5,524 |
Amounts recognized in regulatory assets or liabilities: |  |  |
Actuarial (gain) loss | (4,450) | 3,944 |
Prior service credit | (8,109) | (9,390) |
Total | $ (12,559) | $ (5,446) |

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