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Financial Information - ASU 2017-07 (Details) - USD ($) $ in Thousands | 3 Months Ended | 9 Months Ended | 12 Months Ended
--- | --- | --- | ---
Sep. 30, 2017 | Sep. 30, 2017 | Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015
Employee Compensation, Incentives, and Benefits Expense | Accounting Standards Update 2017-07 |  |  |  |  |
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |  |  |  |
Net periodic benefit cost reclassified | $ 415 | $ 1,665 | $ 1,946 | $ (843) | $ (1,168)

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