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Pension Plans and Other Postretirement Benefits - Narrative (Details) - USD ($) $ in Thousands | 12 Months Ended |  |
--- | --- | --- | ---
Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015 | Dec. 31, 2014 | Sep. 30, 2016 | Dec. 31, 2006
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |  |
Pension cost (credit) | $ 9,114 | $ 8,527 | $ 8,899 |  |  |
Health reimbursement account, amount | 1,526 |  |  |  |  | $ 7,310
Health reimbursement accounts funding | 133 | $ 218 | $ 523 |  |  |
Defined Benefit Plan |  |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |  |
Expected amortization of gain (loss) next year | 371 |  |  |  |  |
Supplemental Defined Benefit Plans |  |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |  |
Expected amortization of gain (loss) next year | $ 310 |  |  |  |  |
Qualified plan | Defined Benefit Plan |  |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |  |
Award granted, amount |  |  |  |  | $ 1,400 |
Qualified plan | 401(k) plan | Defined Benefit Plan |  |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |  |
Vested percentage of contributions | 100.00% |  |  |  |  |
Employer matching contribution as percent of employees' gross pay | 3.00% |  |  |  |  |
Vesting period for Company contributions | 5 years |  |  |  |  |
Qualified plan | Qualified defined contribution plan | Defined Benefit Plan |  |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |  |
Vested percentage of contributions |  |  | 100.00% |  |  |
Employer contribution as percentage of eligible compensation |  |  |  | 5.00% |  |
Minimum years of service required |  |  |  | 1 year |  |
Vesting period for Company contributions |  |  |  | 3 years |  |
Equity securities |  |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |  |
Defined Benefit Plan, Plan Assets, Target Allocation, Percentage | 50.00% |  |  |  |  |
Fixed income funds |  |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |  |
Defined Benefit Plan, Plan Assets, Target Allocation, Percentage | 50.00% |  |  |  |  |

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