Long-Term Debt - Summary of Components of Long-Term Debt Obligations (Detail) - USD ($)	Dec. 31, 2017	Dec. 31, 2016
Debt Instrument [Line Items]
Total debt	$ 400,000,000	$ 370,000,000
Less: unamortized deferred debt issue costs	(3,716,000)	(4,472,000)
Long-term debt, net	396,284,000	365,528,000
Long-term debt, current portion	0	0
Senior Notes | 6% senior notes due 2022
Debt Instrument [Line Items]
Total debt	$ 300,000,000	300,000,000
Interest rate (as a percent)	6.00%
Line of Credit | Credit facility
Debt Instrument [Line Items]
Total debt	$ 100,000,000	70,000,000
Less: unamortized deferred debt issue costs	$ (3,100,000)	$ (4,300,000)