Fair Value Measurements - Additional Information (Detail) - USD ($)	Jun. 30, 2018	Dec. 31, 2017	Sep. 30, 2014
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Aggregate amount of senior notes issued	$ 125,000,000	$ 125,000,000
2014 Notes | 2014 Convertible Notes
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Aggregate amount of senior notes issued	125,000,000	125,000,000.0	$ 125,000,000
Senior secured convertible notes	$ 361,700,000	$ 327,600,000