Convertible Senior Notes - Components of the Interest Expense Recognized (Detail) - USD ($) $ in Thousands	3 Months Ended	6 Months Ended
Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Debt Disclosure [Abstract]
Contractual coupon interest expense	$ 279	$ 757	$ 558	$ 1,513
Amortization of debt issuance costs	62	155	124	321
Accretion of debt discount	545	1,412	1,079	2,794
Total	$ 886	$ 2,324	$ 1,761	$ 4,628
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