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Fair Value of Financial Instruments - Summary of Change in the Estimated Fair Value of Private Placement Warrants (Details) - Warrants - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Dec. 31, 2014
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation        
Balance at beginning of year $ 0 $ 1,482 $ 11,036 $ 24,525
Issuance of warrants (20,700) 0 0 0
Increase (decrease) in value 4,000 (1,413) (9,299) (6,169)
Balance at end of year 24,700 0 1,482 11,036
Private placement warranty liability        
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation        
Issuance of warrants $ 0 $ (69) $ (255) $ (7,320)