Debt and Warrants - Summary of Interest Expense (Detail) - USD ($) $ in Thousands	3 Months Ended	9 Months Ended
Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Interest Expense [Abstract]
Coupon interest expense	$ 665	$ 517	$ 1,770	$ 1,557
Noncash interest expense
Amortization of debt discount	397	393	1,202	1,143
Amortization of transaction costs	68	60	197	174
Miscellaneous interest expense			3	8
Interest expense	$ 1,130	$ 970	$ 3,172	$ 2,882

Amount of noncash expense included in interest expense to amortize debt discount and premium associated with the related debt instruments. Excludes amortization of financing costs. Alternate captions include noncash interest expense.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
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Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
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Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
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-Name Accounting Standards Codification
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