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Components of net periodic benefit cost (Details 3) - Underfunded Plan [Member] - Qualified Plan [Member] - USD ($) $ in Thousands | 12 Months Ended
--- | ---
Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015
Pension Benefits |  |  |
Components of net periodic benefit cost (credit): |  |  |
Service cost | $ 0 | $ 0 | $ 86
Interest cost | 16,207 | 17,218 | 17,141
Expected return on assets | (20,528) | (20,924) | (22,254)
Amortization of prior service cost (credit) | 0 | 0 | 36
Recognized net actuarial loss | 6,355 | 6,215 | 7,016
Curtailment loss | 0 | 0 | 258
Net periodic benefit cost (credit), including amount capitalized | 2,034 | 2,509 | 2,283
Less amount capitalized | 310 | 381 | 316
Net periodic benefit cost (credit) | 1,724 | 2,128 | 1,967
Other changes in plan assets and benefit obligations recognized in accumulated comprehensive (income) loss or regulatory assets (liabilities): |  |  |
Net (gain) loss | (5,827) | (3,789) | 8,257
Amortization of actuarial loss | (6,355) | (6,215) | (7,016)
Amortization of prior service (cost) credit | 0 | 0 | (294)
Total recognized in accumulated other comprehensive (income) loss or regulatory assets (liabilities) | (12,182) | (10,004) | 947
Total recognized in net periodic benefit cost (credit), accumulated other comprehensive (income) loss and regulatory assets (liabilities) | (10,458) | (7,876) | 2,914
Other Postretirement Benefits |  |  |
Components of net periodic benefit cost (credit): |  |  |
Service cost | 1,508 | 1,647 | 1,816
Interest cost | 3,265 | 3,688 | 3,607
Expected return on assets | (4,641) | (4,533) | (4,795)
Amortization of prior service cost (credit) | (1,371) | (1,371) | (1,371)
Recognized net actuarial loss | 857 | 1,491 | 1,960
Curtailment loss | 0 | 0 | 0
Net periodic benefit cost (credit), including amount capitalized | (382) | 922 | 1,217
Less amount capitalized | (370) | (52) | 120
Net periodic benefit cost (credit) | (12) | 974 | 1,097
Other changes in plan assets and benefit obligations recognized in accumulated comprehensive (income) loss or regulatory assets (liabilities): |  |  |
Net (gain) loss | (3,190) | (3,523) | (1,336)
Amortization of actuarial loss | (857) | (1,491) | (1,960)
Amortization of prior service (cost) credit | 1,371 | 1,371 | 1,371
Total recognized in accumulated other comprehensive (income) loss or regulatory assets (liabilities) | (2,676) | (3,643) | (1,925)
Total recognized in net periodic benefit cost (credit), accumulated other comprehensive (income) loss and regulatory assets (liabilities) | $ (2,688) | $ (2,669) | $ (828)

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- Definition Amount, before tax, after reclassification adjustment, of (increase) decrease in accumulated other comprehensive income or regulatory assets (liabilities) for defined benefit plan. + References No definition available. + Details
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- Definition Amount recognized in net periodic benefit costs (credit) and other comprehensive income or regulatory assets (liabilities). + References No definition available. + Details
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- Definition The total amount of net periodic benefit cost for defined benefit plans capitalized for the period. Periodic benefit costs include the following components: service cost, interest cost, expected return on plan assets, gain or loss, prior service cost or credit, transition asset or obligation, and gain or loss due to settlements or curtailments). + References No definition available. + Details
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- Definition The total amount of net periodic benefit cost including amount capitalized for defined benefit plans for the period. Periodic benefit costs include the following components: service cost, interest cost, expected return on plan assets, gain or loss, prior service cost or credit, transition asset or obligation, and gain or loss due to settlements or curtailments), including amount capitalized. + References No definition available. + Details
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- Definition Amount, before tax, of gain (loss) for (increase) decrease in value of benefit obligation for change in actuarial assumptions and increase (decrease) in value of plan assets from experience different from that assumed of defined benefit plan, that has not been recognized in net periodic benefit (cost) credit. + References No definition available. + Details
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- Definition Amount, before tax, of reclassification adjustment from accumulated other comprehensive income (loss) or regulatory (assets) liabilities for gain (loss) of defined benefit plan. + References No definition available. + Details
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- Definition Amount, before tax, of reclassification adjustment from accumulated other comprehensive (income) loss or regulatory assets (liabilities) for prior service cost (credit) of defined benefit plan. + References No definition available. + Details
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- Definition Amount of gain (loss) recognized in net periodic benefit (cost) credit of defined benefit plan. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (h)(4) -URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920 + Details
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