Stock Option Activity (Detail) (USD $) In Thousands, except Share data, unless otherwise specified	9 Months Ended
	Sep. 30, 2012
Number of options
Options outstanding beginning of year	6,595,136
Options granted	1,443,810
Options exercised	(831,957)
Options forfeited/cancelled	(168,595)
Options outstanding end of year	7,038,394
Options vested and exercisable end of year	3,794,531
Options expected to vest	3,026,253
Weighted-Average Exercise Price
Options outstanding beginning of year	$ 4.65
Options granted	$ 16.80
Options exercised	$ 3.74
Options forfeited/cancelled	$ 10.68
Options outstanding at end of year	$ 7.11
Options vested and exercisable end of year	$ 3.32
Options expected to vest	$ 11.40
Aggregate Intrinsic Value
Options outstanding at beginning of year	$ 48,447
Options granted	3,090
Options exercised	(12,646)
Options forfeited/cancelled	(1,393)
Options outstanding at end of year	83,264
Options vested and exercisable at end of year	59,271
Options expected to vest	$ 22,818