Fair Value Measurements - Additional Information (Detail) (USD $) - USD ($)	9 Months Ended	12 Months Ended
	Sep. 30, 2015	Dec. 31, 2014	Sep. 30, 2014	Mar. 18, 2013	May 23, 2011
Impaired Long Lived Assets Held And Used [Line Items]
Impairment of long-lived assets	$ 0	$ 0
Convertible Notes | 2.50% Senior Subordinated Notes Due June 2016
Impaired Long Lived Assets Held And Used [Line Items]
Debt instrument, interest rate, stated percentage	2.50%	2.50%	2.50%		2.50%
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%