Convertible Senior Notes - Components of the Interest Expense Recognized (Detail) - USD ($) $ in Thousands	3 Months Ended	9 Months Ended
Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Debt Disclosure [Abstract]
Contractual coupon interest expense	$ 246	$ 757	$ 798	$ 2,270
Amortization of debt issuance costs	56	170	178	491
Accretion of debt discount	491	1,442	1,558	4,236
Total	$ 793	$ 2,369	$ 2,534	$ 6,997
Effective interest rate	8.41%	8.65%	8.41%	8.65%

Amount of the cost of borrowed funds accounted for as interest expense.

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
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Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration