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Fair Value Measurements - Roll Forward of Level 3 Financial Instruments Measured at Fair Value on a Recurring Basis (Details) - USD ($)
$ in Thousands
3 Months Ended
Mar. 31, 2018
Mar. 31, 2017
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]    
Balance, at beginning of period $ (26,372) $ (25,134)
Settlements 0 0
Total unrealized income (loss) included in earnings 7,087 9,919
Balance, at end of period $ (19,285) $ (15,215)