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Employee Benefit Plans - Postretirement Healthcare Plan (Details) - USD ($) $ in Thousands | 12 Months Ended |
--- | --- | ---
Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015 | Dec. 31, 2014
Defined Benefit Plan Disclosure |  |  |  |
Amortization of gain, weighted average remaining service period (in years) |  | 4 years |  |
Stockholders’ equity | $ 1,427,616 | $ 982,593 | $ 950,493 | $ 908,924
Postretirement Healthcare Plan |  |  |  |
Defined Benefit Plan Disclosure |  |  |  |
Minimum age requirement | 55 years |  |  |
Requisite service period | 15 years |  |  |
Accumulated other comprehensive income (loss), before tax, related to the Plan | $ 2,200 | 2,800 |  |
Stockholders’ equity | 1,300 |  |  |
Reduction in accumulated post-retirement benefit obligation due to curtailment |  | 2,800 |  |
Reduction in net periodic benefit cost due to curtailment |  | 300 |  |
Unfunded benefit obligation | 700 | 700 |  |
Net periodic benefit cost | $ 400 | $ 200 | $ 400 |
Assumptions used calculating net periodic benefit cost, discount rate | 2.40% | 2.40% |  |
Assumptions used calculating net periodic benefit cost, rate of compensation increase | 6.00% | 6.00% |  |
Defined Benefit Plan, Expected Future Benefit Payment [Abstract] |  |  |  |
2018 | $ 130 |  |  |
2019 | 130 |  |  |
2020 | 100 |  |  |
2021 | 70 |  |  |
2022 | 50 |  |  |
2023 through 2027 | 100 |  |  |
Net actuarial gains | 2,100 |  |  |
Unamortized negative prior service costs | 100 |  |  |
Estimated amount amortized from accumulated other comprehensive loss into net periodic benefit costs | $ 800 |  |  |

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- Definition Defined Benefit Plan with Individual, Minimum Age Requirement + References No definition available. + Details
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- Definition Amount, before tax, of accumulated other comprehensive (income) loss for defined benefit plan, that has not been recognized in net periodic benefit cost (credit). + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 220 -SubTopic 10 -Section 45 -Paragraph 14A -URI http://asc.fasb.org/extlink&oid=84228862&loc=SL7669686-108580 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 220 -SubTopic 10 -Section 45 -Paragraph 10A -Subparagraph (i-k) -URI http://asc.fasb.org/extlink&oid=84228862&loc=SL7669646-108580 + Details
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- Definition Amount, before tax, of accumulated other comprehensive (income) loss 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 Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 220 -SubTopic 10 -Section 45 -Paragraph 14A -URI http://asc.fasb.org/extlink&oid=84228862&loc=SL7669686-108580 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (j) -URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920 + Details
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- Definition Amount, before tax, of accumulated other comprehensive income (loss) for transition asset (obligation) of defined benefit plan, that has not been recognized in net periodic benefit (cost) credit. + 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 (j) -URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 220 -SubTopic 10 -Section 45 -Paragraph 14A -URI http://asc.fasb.org/extlink&oid=84228862&loc=SL7669686-108580 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 220 -SubTopic 10 -Section 45 -Paragraph 10A -Subparagraph (k) -URI http://asc.fasb.org/extlink&oid=84228862&loc=SL7669646-108580 + Details
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- Definition Weighted average rate of compensation increase used to determine net periodic benefit cost of defined benefit plan. Plan includes, but is not limited to, pay-related 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 (k)(2) -URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920 + Details
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