Activity Under Stock Option Plans (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	9 Months Ended
	Sep. 30, 2012
Number of Shares Underlying Stock Options
Outstanding as of December 31, 2011	6,190
Granted	0	[1]
Exercised	(2,635)
Share-based Compensation Arrangements by Share-based Payment Award, Options, Exercises in Period, Weighted Average Exercise Price
Cancelled or expired	(29)
Outstanding as of September 30, 2012	3,526
Vested and expected to vest at September 30, 2012	3,483
Exercisable at June 30, 2012	2,788
Weighted Average Exercise Price per Share
Outstanding as of September 30, 2012 (in usd per share)	$ 15.17
Vested and expected to vest at September 30, 2012 (in usd per share)	$ 15.12
Exercisable at September 30, 2012 (in usd per share)	$ 14.51
Weighted Average Remaining Contractual Term
Outstanding as of September 30, 2012	4 years 7 months 24 days
Vested and expected to vest at September 30, 2012	4 years 7 months 24 days
Exercisable as September 30, 2012	4 years 6 months 18 days
Aggregate Intrinsic Value
Outstanding as of September 30, 2012	$ 76,880
Vested and expected to vest at September 30, 2012	76,098
Exercisable at September 30, 2012	$ 62,627

[1] There were no stock options granted during the three months ended September 30, 2012 and 2011 and for the nine months ended September 30, 2012.