Summary of Borrowings (Detail) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017	Oct. 23, 2017
Debt Instrument [Line Items]
Principal amount	$ 140,200
Note payable to related party - net carrying amount	72,247	$ 79,666
Total debt - net carrying amount	140,269	156,822
Tranche B Notes
Debt Instrument [Line Items]
Principal amount	10,000
Senior convertible notes due December 31, 2019
Debt Instrument [Line Items]
Principal amount	35,000
Senior convertible notes due December 31, 2019 | Tranche B Notes
Debt Instrument [Line Items]
Principal amount	45,000	54,407
Unamortized debt issuance costs and debt discount	(1,346)	(1,662)
Net carrying amount	43,654	52,745
5.75% Senior convertible notes due October 23, 2021
Debt Instrument [Line Items]
Principal amount	23,690	23,690	$ 23,700
Unamortized premium	678	721
Net carrying amount	$ 24,368	$ 24,411