Stock Option Activity (Detail) (USD $)	6 Months Ended
	Jun. 30, 2013
Number of Shares
Options outstanding, January 1, 2013	8,228,334
Granted	2,812,015
Forfeited	(69,839)
Exercised	(330,790)
Options outstanding, June 30, 2013	10,639,720
Weighted Average Exercise Price Per Share
Options outstanding, January 1, 2013	$ 10.20
Granted	$ 18.68
Forfeited	$ 16.86
Exercised	$ 5.10
Options outstanding, June 30, 2013	$ 12.55