Fair Value Measurements - Roll-Forward of Fair Value of Derivative Liability (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Change in fair value of derivative liability, recorded as interest expense	$ 83	$ (142)
Probability-Weighted Discounted Cash Flow Model [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Balance at December 31, 2018	2,142
Change in fair value of derivative liability, recorded as interest expense	83
Balance at March 31, 2019	$ 2,225