Summary of Stock Option Activity and Weighted Average Exercise Prices (Detail) (USD $)	6 Months Ended
	Jun. 30, 2012
Options
Options outstanding at December 31, 2011	8,843,413
Options granted	1,120,300
Options exercised	(1,388,890)
Options cancelled	(363,083)
Options outstanding at June 30, 2012	8,211,740
Options exercisable at June 30,2012	2,774,457
Weighted Average Exercise Price
Options outstanding at December 31, 2011	$ 7.91
Options granted	$ 14.15
Options exercised	$ 4.81
Options cancelled	$ 9.00
Options outstanding at June 30, 2012	$ 9.24
Options exercisable at June 30,2012	$ 5.89