Fair Value of Financial Assets and Liabilities - Summary of Changes in Fair Value of Derivative Liability for Fair Value Determined by Level 3 Inputs (Detail) - Level 3 [Member] - Derivative Liabilities [Member] $ in Thousands	6 Months Ended
	Jun. 30, 2018 USD ($)
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning balance	$ 371
Change in fair value	1,629
Transferred to accrued expenses	$ (2,000)