Fair Value Measurements - Additional Information (Detail) - USD ($)	Mar. 31, 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
Long-term debt, fair value	$ 299,000,000	327,600,000
2014 Notes | 2014 Convertible Notes
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Aggregate amount of senior notes issued		$ 125,000,000.0	$ 125,000,000