Stockholders' Equity (Stock Option Plan Activity) (Details) (USD $) In Thousands, except Share data, unless otherwise specified	9 Months Ended	12 Months Ended
	Sep. 30, 2013	Dec. 31, 2012
Shares
Balance as of December 31, 2012	1,626,606
Granted	439,929
Exercised	(158,889)
Forfeited	(73,978)
Cancelled	(27,780)
Balance as of September 30, 2013	1,805,888	1,626,606
Exercisable as of September 30, 2013	943,027
Options vested and expected to vest as of September 30, 2013	1,805,888
Weighted Average Exercise Price
Balance as of December 31, 2012 (in usd per share)	$ 14.90
Granted (in usd per share)	$ 23.61
Exercised (in usd per share)	$ 14.76
Forfeited (in usd per share)	$ 16.72
Cancelled (in usd per share)	$ 28.32
Balance as of September 30, 2013 (in usd per share)	$ 16.74	$ 14.90
Exercisable as of September 30, 2013 (in usd per share)	$ 15.66
Options vested and expected to vest as of June 30, 2013 (in usd per share)	$ 16.74
Weighted Average Remaining Contractual Term in Years
Weighted average remaining contractual term in years	7 years 8 months 15 days	7 years 9 months 29 days
Exercisable as of September 30, 2013	6 years 9 months 25 days
Options vested and expected to vest as of September 30, 2013	7 years 8 months 15 days
Aggregate Intrinsic Value
Aggregate intrinsic value	$ 48,322	$ 11,358
Exercisable as of September 30, 2013	26,419
Options vested and expected to vest as of September 30, 2013	$ 48,322