Summary of Borrowings (Detail) - USD ($)	Dec. 31, 2017	Oct. 23, 2017	Dec. 31, 2016
Debt Instrument [Line Items]
Principal amount	$ 157,800,000
Note payable to principal stockholder - net carrying amount	79,666,000		$ 49,521,000
Senior convertible notes due December 31, 2019 | Tranche B Notes
Debt Instrument [Line Items]
Principal amount	54,407,000		75,000,000
Unamortized debt issuance costs and debt discount	(1,662,000)		(3,661,000)
Net carrying amount	52,745,000		71,339,000
5.75% Senior convertible notes due October 23, 2021
Debt Instrument [Line Items]
Principal amount	23,690,000	$ 23,690,000	27,690,000
Unamortized premium	721,000	$ 800,000	426,000
Unaccreted debt issuance costs			(481,000)
Net carrying amount	$ 24,411,000		$ 27,635,000