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Defined Benefit Plan (Details) - USD ($) $ in Thousands | 3 Months Ended | 6 Months Ended |
--- | --- | --- | ---
Jun. 30, 2019 | Jun. 30, 2018 | Jun. 30, 2019 | Jun. 30, 2018 | Dec. 31, 2018
Net Periodic Benefit Cost - Comprehensive Income Presentation [Abstract] |  |  |  |  |  |  |  |
Other production expenses | $ 370 |  | $ 426 | $ 740 |  | $ 849 |  |
Administrative and general expense | 851 |  | 902 | 1,689 |  | 1,797 |  |
Maintenance expense | 271 |  | 279 | 542 |  | 557 |  |
Pension non-service costs | 907 |  | 595 | 1,828 |  | 1,178 |  |
Components of Net Periodic Benefit Cost [Abstract] |  |  |  |  |  |  |  |
Service cost | 1,479 |  | 1,607 | 2,958 |  | 3,203 |  |
Interest cost | 2,113 |  | 1,876 | 4,225 |  | 3,753 |  |
Other cost | 1,117 |  | 1,145 | 2,235 |  | 2,278 |  |
Expected return on assets | (2,310) |  | (2,426) | (4,619) |  | (4,853) |  |
Net periodic benefit cost | 2,399 |  | $ 2,202 | 4,799 |  | $ 4,381 |  |
Plan Assets [Abstract] |  |  |  |  |  |  |  |
Fair value of plan assets | 152,539 |  |  | 152,539 |  |  | $ 133,459 |
Employer Contributions [Abstract] |  |  |  |  |  |  |  |
Estimated employer contributions for the current fiscal year | 8,337 |  |  | 8,337 |  |  |  |
Defined Contribution Plan, Employer Discretionary Contribution Amount | 2,294 |  |  |  |  |  |  |
Quoted Prices in Active Markets for Identical Assets |  |  |  |  |  |  |  |
Plan Assets [Abstract] |  |  |  |  |  |  |  |
Fair value of plan assets | 97,878 |  |  | 97,878 |  |  | 82,677 |
Significant Observable Inputs |  |  |  |  |  |  |  |
Plan Assets [Abstract] |  |  |  |  |  |  |  |
Fair value of plan assets | 54,661 |  |  | 54,661 |  |  | 50,782 |
Significant Unobservable Inputs |  |  |  |  |  |  |  |
Plan Assets [Abstract] |  |  |  |  |  |  |  |
Fair value of plan assets | $ 0 |  |  | $ 0 |  |  | 0 |
Equity Securities [Member] |  |  |  |  |  |  |  |
Plan Assets [Abstract] |  |  |  |  |  |  |  |
Target plan asset allocations | 55.00% |  |  | 55.00% |  |  |  |
Fixed Income Securities |  |  |  |  |  |  |  |
Plan Assets [Abstract] |  |  |  |  |  |  |  |
Target plan asset allocations | 45.00% |  |  | 45.00% |  |  |  |
Cash and cash equivalents |  |  |  |  |  |  |  |
Plan Assets [Abstract] |  |  |  |  |  |  |  |
Fair value of plan assets | $ 7,723 |  |  | $ 7,723 |  |  | 8,136 |
Cash and cash equivalents | Quoted Prices in Active Markets for Identical Assets |  |  |  |  |  |  |  |
Plan Assets [Abstract] |  |  |  |  |  |  |  |
Fair value of plan assets | 7,723 |  |  | 7,723 |  |  | 8,136 |
Cash and cash equivalents | Significant Observable Inputs |  |  |  |  |  |  |  |
Plan Assets [Abstract] |  |  |  |  |  |  |  |
Fair value of plan assets | 0 |  |  | 0 |  |  | 0 |
Cash and cash equivalents | Significant Unobservable Inputs |  |  |  |  |  |  |  |
Plan Assets [Abstract] |  |  |  |  |  |  |  |
Fair value of plan assets | 0 |  |  | 0 |  |  | 0 |
Actively Managed | All Cap Equity |  |  |  |  |  |  |  |
Plan Assets [Abstract] |  |  |  |  |  |  |  |
Fair value of plan assets | 6,461 | [1] |  | 6,461 | [1] |  | 5,670 | [2]
Actively Managed | All Cap Equity | Quoted Prices in Active Markets for Identical Assets |  |  |  |  |  |  |  |
Plan Assets [Abstract] |  |  |  |  |  |  |  |
Fair value of plan assets | 6,426 | [1] |  | 6,426 | [1] |  | 5,632 | [2]
Actively Managed | All Cap Equity | Significant Observable Inputs |  |  |  |  |  |  |  |
Plan Assets [Abstract] |  |  |  |  |  |  |  |
Fair value of plan assets | 35 | [1] |  | 35 | [1] |  | 38 | [2]
Actively Managed | All Cap Equity | Significant Unobservable Inputs |  |  |  |  |  |  |  |
Plan Assets [Abstract] |  |  |  |  |  |  |  |
Fair value of plan assets | 0 | [1] |  | 0 | [1] |  | 0 | [2]
Actively Managed | U.S. Large Cap Equity |  |  |  |  |  |  |  |
Plan Assets [Abstract] |  |  |  |  |  |  |  |
Fair value of plan assets | 57,375 | [1] |  | 57,375 | [1] |  | 47,040 | [2]
Actively Managed | U.S. Large Cap Equity | Quoted Prices in Active Markets for Identical Assets |  |  |  |  |  |  |  |
Plan Assets [Abstract] |  |  |  |  |  |  |  |
Fair value of plan assets | 57,375 | [1] |  | 57,375 | [1] |  | 47,040 | [2]
Actively Managed | U.S. Large Cap Equity | Significant Observable Inputs |  |  |  |  |  |  |  |
Plan Assets [Abstract] |  |  |  |  |  |  |  |
Fair value of plan assets | 0 | [1] |  | 0 | [1] |  | 0 | [2]
Actively Managed | U.S. Large Cap Equity | Significant Unobservable Inputs |  |  |  |  |  |  |  |
Plan Assets [Abstract] |  |  |  |  |  |  |  |
Fair value of plan assets | 0 | [1] |  | 0 | [1] |  | 0 | [2]
Actively Managed | U.S. Mid Cap Equity |  |  |  |  |  |  |  |
Plan Assets [Abstract] |  |  |  |  |  |  |  |
Fair value of plan assets | 10,202 | [1] |  | 10,202 | [1] |  | 8,372 | [2]
Actively Managed | U.S. Mid Cap Equity | Quoted Prices in Active Markets for Identical Assets |  |  |  |  |  |  |  |
Plan Assets [Abstract] |  |  |  |  |  |  |  |
Fair value of plan assets | 10,202 | [1] |  | 10,202 | [1] |  | 8,372 | [2]
Actively Managed | U.S. Mid Cap Equity | Significant Observable Inputs |  |  |  |  |  |  |  |
Plan Assets [Abstract] |  |  |  |  |  |  |  |
Fair value of plan assets | 0 | [1] |  | 0 | [1] |  | 0 | [2]
Actively Managed | U.S. Mid Cap Equity | Significant Unobservable Inputs |  |  |  |  |  |  |  |
Plan Assets [Abstract] |  |  |  |  |  |  |  |
Fair value of plan assets | 0 | [1] |  | 0 | [1] |  | 0 | [2]
Actively Managed | U.S. Small Cap Equity |  |  |  |  |  |  |  |
Plan Assets [Abstract] |  |  |  |  |  |  |  |
Fair value of plan assets | 10,385 | [1] |  | 10,385 | [1] |  | 8,528 | [2]
Actively Managed | U.S. Small Cap Equity | Quoted Prices in Active Markets for Identical Assets |  |  |  |  |  |  |  |
Plan Assets [Abstract] |  |  |  |  |  |  |  |
Fair value of plan assets | 10,385 | [1] |  | 10,385 | [1] |  | 8,528 | [2]
Actively Managed | U.S. Small Cap Equity | Significant Observable Inputs |  |  |  |  |  |  |  |
Plan Assets [Abstract] |  |  |  |  |  |  |  |
Fair value of plan assets | 0 | [1] |  | 0 | [1] |  | 0 | [2]
Actively Managed | U.S. Small Cap Equity | Significant Unobservable Inputs |  |  |  |  |  |  |  |
Plan Assets [Abstract] |  |  |  |  |  |  |  |
Fair value of plan assets | 0 | [1] |  | 0 | [1] |  | 0 | [2]
Actively Managed | Non-U.S. Large Cap Equity |  |  |  |  |  |  |  |
Plan Assets [Abstract] |  |  |  |  |  |  |  |
Fair value of plan assets | 5,767 | [1] |  | 5,767 | [1] |  | 4,969 | [2]
Actively Managed | Non-U.S. Large Cap Equity | Quoted Prices in Active Markets for Identical Assets |  |  |  |  |  |  |  |
Plan Assets [Abstract] |  |  |  |  |  |  |  |
Fair value of plan assets | 5,767 | [1] |  | 5,767 | [1] |  | 4,969 | [2]
Actively Managed | Non-U.S. Large Cap Equity | Significant Observable Inputs |  |  |  |  |  |  |  |
Plan Assets [Abstract] |  |  |  |  |  |  |  |
Fair value of plan assets | 0 | [1] |  | 0 | [1] |  | 0 | [2]
Actively Managed | Non-U.S. Large Cap Equity | Significant Unobservable Inputs |  |  |  |  |  |  |  |
Plan Assets [Abstract] |  |  |  |  |  |  |  |
Fair value of plan assets | 0 | [1] |  | 0 | [1] |  | 0 | [2]
Actively Managed | REIT |  |  |  |  |  |  |  |
Plan Assets [Abstract] |  |  |  |  |  |  |  |
Fair value of plan assets | 7,137 | [1] |  | 7,137 | [1] |  | 5,889 | [2]
Actively Managed | REIT | Quoted Prices in Active Markets for Identical Assets |  |  |  |  |  |  |  |
Plan Assets [Abstract] |  |  |  |  |  |  |  |
Fair value of plan assets | 0 | [1] |  | 0 | [1] |  | 0 | [2]
Actively Managed | REIT | Significant Observable Inputs |  |  |  |  |  |  |  |
Plan Assets [Abstract] |  |  |  |  |  |  |  |
Fair value of plan assets | 7,137 | [1] |  | 7,137 | [1] |  | 5,889 | [2]
Actively Managed | REIT | Significant Unobservable Inputs |  |  |  |  |  |  |  |
Plan Assets [Abstract] |  |  |  |  |  |  |  |
Fair value of plan assets | 0 | [1] |  | 0 | [1] |  | 0 | [2]
Fixed Income |  |  |  |  |  |  |  |
Plan Assets [Abstract] |  |  |  |  |  |  |  |
Fair value of plan assets | 47,489 | [3] |  | 47,489 | [3] |  | 44,855 | [4]
Fixed Income | Quoted Prices in Active Markets for Identical Assets |  |  |  |  |  |  |  |
Plan Assets [Abstract] |  |  |  |  |  |  |  |
Fair value of plan assets | 0 | [3] |  | 0 | [3] |  | 0 | [4]
Fixed Income | Significant Observable Inputs |  |  |  |  |  |  |  |
Plan Assets [Abstract] |  |  |  |  |  |  |  |
Fair value of plan assets | 47,489 | [3] |  | 47,489 | [3] |  | 44,855 | [4]
Fixed Income | Significant Unobservable Inputs |  |  |  |  |  |  |  |
Plan Assets [Abstract] |  |  |  |  |  |  |  |
Fair value of plan assets | $ 0 | [3] |  | $ 0 | [3] |  | $ 0 | [4]
[1] | Actively managed portfolio of securities with the goal to exceed the stated benchmark performance.
[2] | Actively managed portfolio of securities with the goal to exceed the stated benchmark performance.
[3] | Actively managed portfolio of fixed income securities with the goal to exceed the Barclays 1-5 Year Government/Credit, Barclays Intermediate Government/Credit, and Merrill Lynch Preferred Stock Fixed Rate.
[4] | Actively managed portfolio of fixed income securities with the goal to exceed the Barclays 1-5 Year Government/Credit, Barclays Intermediate Government/Credit, and Merrill Lynch Preferred Stock Fixed Rate.

X
- Definition The employer's best estimate of total contributions expected to be paid to the plan for the current fiscal period. Estimated contributions may be presented in the aggregate combining (1) contributions required by funding regulations or laws, (2) discretionary contributions, and (3) noncash contributions. + References No definition available. + Details
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- Definition Defined Benefit Plan, General and Administrative Cost + References No definition available. + Details
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- Definition Defined Benefit Plan, Net Periodic Benefit Cost, Comprehensive Income Presentation [Abstract] + References No definition available. + Details
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- Definition Amount of expected return (loss) recognized in net periodic benefit (cost) credit, calculated based on expected long-term rate of return and market-related value of plan assets of defined benefit plan. + References Reference 1: http://www.xbrl.org/2003/role/exampleRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 55 -Paragraph 17 -URI http://asc.fasb.org/extlink&oid=112275985&loc=d3e4179-114921 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (h)(3) -URI http://asc.fasb.org/extlink&oid=108410482&loc=d3e1928-114920 Reference 3: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 6 -Subparagraph (a)(3) -URI http://asc.fasb.org/extlink&oid=108410482&loc=d3e2709-114920 + Details
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- Definition Amount of asset segregated and restricted to provide benefit under defined benefit plan. Asset includes, but is not limited to, stock, bond, other investment, earning from investment, and contribution by employer and employee. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (d)(iv)(01) -URI http://asc.fasb.org/extlink&oid=108410482&loc=d3e1928-114920 Reference 2: http://www.xbrl.org/2003/role/exampleRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (d)(iv)(01) -URI http://asc.fasb.org/extlink&oid=108410482&loc=d3e1928-114920 Reference 3: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (d)(ii) -URI http://asc.fasb.org/extlink&oid=108410482&loc=d3e1928-114920 Reference 4: http://www.xbrl.org/2003/role/exampleRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 55 -Paragraph 17 -URI http://asc.fasb.org/extlink&oid=112275985&loc=d3e4179-114921 Reference 5: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=108410482&loc=d3e1928-114920 Reference 6: http://www.xbrl.org/2003/role/exampleRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (d)(ii) -URI http://asc.fasb.org/extlink&oid=108410482&loc=d3e1928-114920 + Details
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- Definition Amount of cost recognized for passage of time related to defined benefit plan. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (h)(2) -URI http://asc.fasb.org/extlink&oid=108410482&loc=d3e1928-114920 Reference 2: http://www.xbrl.org/2003/role/exampleRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 55 -Paragraph 17 -URI http://asc.fasb.org/extlink&oid=112275985&loc=d3e4179-114921 Reference 3: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (a)(2) -URI http://asc.fasb.org/extlink&oid=108410482&loc=d3e1928-114920 Reference 4: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 6 -Subparagraph (a)(2) -URI http://asc.fasb.org/extlink&oid=108410482&loc=d3e2709-114920 + Details
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- Definition Amount of net periodic benefit cost (credit) for defined benefit plan. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 6 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=108410482&loc=d3e2709-114920 Reference 2: http://www.xbrl.org/2003/role/exampleRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 55 -Paragraph 17 -URI http://asc.fasb.org/extlink&oid=112275985&loc=d3e4179-114921 Reference 3: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (h) -URI http://asc.fasb.org/extlink&oid=108410482&loc=d3e1928-114920 + Details
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- Definition Percentage of target investment allocation to total plan assets. Includes, but is not limited to, percentage on weighted-average basis if more than one plan. + References Reference 1: http://www.xbrl.org/2003/role/exampleRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 55 -Paragraph 17 -URI http://asc.fasb.org/extlink&oid=112275985&loc=d3e4179-114921 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (d)(i) -URI http://asc.fasb.org/extlink&oid=108410482&loc=d3e1928-114920 + Details
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- Definition Amount of cost for actuarial present value of benefits attributed to service rendered by employee for defined benefit plan. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (h)(1) -URI http://asc.fasb.org/extlink&oid=108410482&loc=d3e1928-114920 Reference 2: http://www.xbrl.org/2003/role/exampleRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 55 -Paragraph 17 -URI http://asc.fasb.org/extlink&oid=112275985&loc=d3e4179-114921 Reference 3: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 6 -Subparagraph (a)(1) -URI http://asc.fasb.org/extlink&oid=108410482&loc=d3e2709-114920 Reference 4: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (a)(1) -URI http://asc.fasb.org/extlink&oid=108410482&loc=d3e1928-114920 + Details
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