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Summary of Significant Accounting Policies - Reconciliation of Assets and Liabilities Measured at Fair Value Using Significant Observable Inputs Level 3 (Detail) - Common Stock Warrant Liabilities - USD ($)
$ in Thousands
3 Months Ended
Mar. 31, 2017
Mar. 31, 2016
Liabilities    
Beginning Balance   $ 6,196
Changes in fair value $ (587) (4,527)
Ending Balance   $ 1,669