Fair Value Measurements - Additional Information (Detail) - USD ($)	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016	Dec. 31, 2016	Mar. 18, 2013	May 23, 2011
Impaired Long Lived Assets Held And Used [Line Items]
Impairment of long-lived assets			$ 0		$ 0
Fair value of available for sale securities	$ 1,300,000		1,300,000		$ 1,900,000
Change in fair value of available for sale securities	46,000	$ 47,000	$ (625,000)	$ (2,179,000)
Maximum
Impaired Long Lived Assets Held And Used [Line Items]
Change in fair value of available for sale securities	$ (100,000)
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%	1.50%	1.50%	2.50%