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Employee Benefit Plans - Fair Value of Plan Assets Roll-Forward (Details)
$ in Thousands
12 Months Ended
Dec. 31, 2017
USD ($)
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]  
Balance at beginning of period $ 0
Purchase of asset 33,155
Change in fair value 398
Balance at end of period $ 33,553