Fair Value Measurements - Additional Information (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Mar. 18, 2013
Impaired Long Lived Assets Held And Used [Line Items]
Impairment of long-lived assets	$ 0	$ 0	$ 0
Fair value of available for sale securities	1,400,000	1,900,000
Decrease in fair value of available for sale securities	$ (484,000)	$ (1,990,000)	$ (703,000)
Convertible Notes | 1.50% Senior Subordinated Notes Due March 15, 2018
Impaired Long Lived Assets Held And Used [Line Items]
Debt instrument, interest rate, stated percentage	1.50%	1.50%	1.50%	1.50%