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Change in benefit obligations and plan assets (Details) - Underfunded Plan [Member] - Qualified Plan [Member] - USD ($) $ in Thousands | 12 Months Ended
--- | ---
Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015
Pension Benefits |  |  |
Change in benefit obligation: |  |  |
Benefit obligation at beginning of year | $ 436,307 | $ 442,960 |
Service cost | 0 | 0 | $ 86
Interest cost | 16,207 | 17,218 | 17,141
Plan participants' contributions | 0 | 0 |
Actuarial (gain) loss | 19,119 | 1,882 |
Benefits paid | (25,710) | (25,753) |
Benefit obligation at end of year | 445,923 | 436,307 | 442,960
Change in net plan assets: |  |  |
Fair value of plan assets at beginning of year | 333,509 | 332,667 |
Actual gain on plan assets | 45,473 | 26,595 |
Employer contribution | 1,112 | 0 |
Plan participants' contributions | 0 | 0 |
Benefits paid | (25,710) | (25,753) |
Fair value of net plan assets at end of year | 354,384 | 333,509 | 332,667
Funded status - under | (91,539) | (102,798) |
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 | 91,539 | 102,798 |
Benefit obligation liabilities - net amount recognized | (91,539) | (102,798) |
Amounts recognized in accumulated other comprehensive (income) loss or regulatory assets (liabilities) consist of: |  |  |
Actuarial loss | 186,486 | 198,668 |
Prior service credit | 0 | 0 |
Total | 186,486 | 198,668 |
Other Postretirement Benefits |  |  |
Change in benefit obligation: |  |  |
Benefit obligation at beginning of year | 89,304 | 92,734 |
Service cost | 1,508 | 1,647 | 1,816
Interest cost | 3,265 | 3,688 | 3,607
Plan participants' contributions | 1,368 | 1,405 |
Actuarial (gain) loss | 1,781 | (3,872) |
Benefits paid | (6,020) | (6,298) |
Benefit obligation at end of year | 91,206 | 89,304 | 92,734
Change in net plan assets: |  |  |
Fair value of plan assets at beginning of year | 82,846 | 82,593 |
Actual gain on plan assets | 9,612 | 4,184 |
Employer contribution | 933 | 962 |
Plan participants' contributions | 1,368 | 1,405 |
Benefits paid | (6,020) | (6,298) |
Fair value of net plan assets at end of year | 88,739 | 82,846 | $ 82,593
Funded status - under | (2,467) | (6,458) |
Amounts recognized in the Consolidated Balance Sheets at December 31: |  |  |
Deferred charges and other assets - other | 19,114 | 13,131 |
Other accrued liabilities | 612 | 538 |
Deferred credits and other liabilities - other | 20,969 | 19,051 |
Benefit obligation liabilities - net amount recognized | (2,467) | (6,458) |
Amounts recognized in accumulated other comprehensive (income) loss or regulatory assets (liabilities) consist of: |  |  |
Actuarial loss | 13,423 | 17,470 |
Prior service credit | (11,632) | (13,003) |
Total | $ 1,791 | $ 4,467 |

X
- Definition Amount, before tax, of accumulated other comprehensive (income) loss or regulatory (assets) liabilities for cost (credit) of benefit change attributable to participants' prior service from plan amendment or plan initiation of defined benefit plan, that has not been recognized in net periodic benefit cost (credit). + References No definition available. + Details
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