Fair Value of Financial Assets and Liabilities - Roll Forward of the Fair values of Derivative Liabilities (Detail) - Level 3 [Member] - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Balance at December 31, 2015	$ 2,708	$ 3,384
Fair value at issuance		3,597
Change in fair value	(1,541)	(580)
Settlement	(944)	(3,693)
Balance at December 31, 2016	223	2,708
Contingent prepayment option [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Balance at December 31, 2015	902
Fair value at issuance		908
Change in fair value	42	(6)
Settlement	(944)
Balance at December 31, 2016		902
Tranche Rights [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Balance at December 31, 2015		2,404
Fair value at issuance		909
Change in fair value		(600)
Settlement		(2,713)
Balance at December 31, 2016
Anti- dilution Rights [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Balance at December 31, 2015	1,806	980
Fair value at issuance		1,780
Change in fair value	(1,583)	26
Settlement		(980)
Balance at December 31, 2016	$ 223	$ 1,806