Fair Value Measurements - Additional Information (Detail) (USD $)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Impaired Long-Lived Assets Held and Used [Line Items]
Impairment of long-lived assets	$ 0	$ 0	$ 0
2.50% Convertible Notes
Impaired Long-Lived Assets Held and Used [Line Items]
Debt instrument, interest rate, stated percentage	2.50%
1.50% Convertible Notes
Impaired Long-Lived Assets Held and Used [Line Items]
Debt instrument, interest rate, stated percentage	1.50%