Fair Value of Financial Assets and Liabilities - Summary of Financial Assets and Liabilities Measured at Fair Value Recurring Basis (Detail) - Fair Value, Measurements, Recurring [Member] - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Cash equivalents:
Assets, Fair Value Disclosure	$ 83,121
Derivative liabilities:
Derivative Liabilities	223	$ 2,708
Money Market Funds [Member]
Cash equivalents:
Assets, Fair Value Disclosure	83,121
Level 2 [Member]
Cash equivalents:
Assets, Fair Value Disclosure	83,121
Level 2 [Member] | Money Market Funds [Member]
Cash equivalents:
Assets, Fair Value Disclosure	83,121
Level 3 [Member]
Derivative liabilities:
Derivative Liabilities	223	2,708
Anti- dilution Rights [Member]
Derivative liabilities:
Derivative liabilities	223	1,806
Anti- dilution Rights [Member] | Level 3 [Member]
Derivative liabilities:
Derivative liabilities	$ 223	1,806
Contingent prepayment option [Member]
Derivative liabilities:
Derivative liabilities		902
Contingent prepayment option [Member] | Level 3 [Member]
Derivative liabilities:
Derivative liabilities		$ 902