Summary of Stock Option Activity (Detail) (USD $)	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Stock Options
Stock Options, Beginning balance	2,099,300	2,039,338	2,080,666
Stock Options, Granted	259,800	224,000	658,000
Stock Options, Exercised	(667,243)	(164,038)	(615,328)
Stock Options, Cancelled	(500)		(84,000)
Stock Options, Ending balance	1,691,357	2,099,300	2,039,338
Stock Options, Exercisable	802,990
Weighted- Average Exercise Price
Weighted-Average Exercise Price, Beginning balance	$ 6.18	$ 5.25	$ 1.83
Weighted-Average Exercise Price, Granted	$ 28.67	$ 20.07	$ 7.86
Weighted-Average Exercise Price, Exercised	$ 2.41	$ 0.29	$ 0.09
Weighted-Average Exercise Price, Cancelled	$ 30.58		$ 4.02
Weighted-Average Exercise Price, Ending balance	$ 11.11	$ 6.18	$ 5.25
Weighted-Average Exercise Price, Exercisable	$ 5.91