Activity Under Stock Option Plans (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	6 Months Ended
	Jun. 30, 2013	Dec. 31, 2011
Number of Shares Underlying Stock Options
Outstanding as of December 31, 2012		3,276
Granted	0
Exercised	(1,181)
Cancelled or expired	(72)
Outstanding as of June 30, 2013	2,023	3,276
Vested and expected to vest at June 30, 2013	2,023
Exercisable at June 30, 2013	1,731
Weighted Average Exercise Price per Share
Outstanding as of June 30, 2013	$ 15.51
Vested and expected to vest at March 31, 2013 (usd per share)	$ 15.51
Exercisable at March 31, 2013 (usd per share)	$ 14.95
Weighted Average Remaining Contractual Term
Outstanding as of June 30, 2013	3 years 10 months 13 days
Vested and expected to vest at June 30, 2013	3 years 10 months 13 days
Exercisable at June 30, 2013	3 years 9 months 22 days
Aggregate Intrinsic Value
Outstanding as of June 30, 2013	$ 43,561
Vested and expected to vest at June 30, 2013	43,561
Exercisable at June 30, 2013	$ 38,241