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Employee Benefits - Additional Information (Details) |  | 12 Months Ended
--- | --- | ---
Dec. 19, 2017 USD ($) retiree | Dec. 31, 2017 USD ($) | Dec. 31, 2016 USD ($) | Dec. 31, 2015 USD ($)
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Number of consecutive years of employee's compensation |  | 5 years |  |
Number of total years of employee's compensation |  | 10 years |  |
Net actuarial loss included in accumulated other comprehensive losses expected to be recognized |  | $ 2,200,000 |  |
Future prior service credit to be recognized due to freeze of accrual benefits |  | 0 |  |
Accumulated benefit obligation |  | 106,700,000 | $ 105,800,000 |
Number of retirees covered by annuity contract | retiree | 124 |  |  |
Number of retirees covered by annuity contract receiving death benefits | retiree | 30 |  |  |
Defined benefit obligations, death benefits | $ 2,000 |  |  |
Total premium paid out of plan's assets | $ 1,500,000 | 8,244,000 | 5,250,000 |
Pension contributions |  | 0 |  |
Projected benefit obligation |  | 106,664,000 | 105,834,000 | $ 109,747,000
Net periodic benefit cost |  | (347,000) | 296,000 | 1,736,000
Transfers of plan assets between levels 1 and 2 on recurring basis |  | 0 | 0 |
Purchases, transfers or sales of level 3 plan assets |  | 0 |  |
Supplemental Executive Retirement Plan |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Projected benefit obligation |  | 5,300,000 | 4,700,000 |
Net periodic benefit cost |  | 500,000 | 500,000 | 600,000
Amounts before tax reflected in accumulated other comprehensive income (loss) |  | $ 2,700,000 | 2,500,000 | 2,100,000
Thrift Plan |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Contributions to the Thrift Plan |  | 3.50% |  |
Compensation expense |  | $ 1,800,000 | $ 1,700,000 | $ 1,500,000
Marketable equity securities | Minimum |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Targeted asset allocation percentage |  | 50.00% |  |
Marketable equity securities | Maximum |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Targeted asset allocation percentage |  | 70.00% |  |
Fixed Income | Minimum |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Targeted asset allocation percentage |  | 30.00% |  |
Fixed Income | Maximum |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Targeted asset allocation percentage |  | 50.00% |  |

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- Definition Number Of Consecutive Periods Of Highest Compensation Used To Determine Defined Benefit Pension Plan Benefits + References No definition available. + Details
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- Definition Amount of actuarial present value of benefits attributed to employee service rendered, excluding assumptions about future compensation level. + 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 (e) -URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920 + Details
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- Definition Amount of payment to participant under defined benefit plan which decreases plan assets. For pension plan, payment includes, but is not limited to, pension benefits and death benefits. For other postretirement plan, payment includes, but is not limited to, prescription drug benefits, health care benefits, life insurance benefits, and legal, educational and advisory services. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 60 -Section 50 -Paragraph 4 -URI http://asc.fasb.org/extlink&oid=6414203&loc=d3e39716-114964 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (b)(5) -URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920 + Details
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