Employee Stock Benefit Plans (Details 2) (Options to purchase common stock, USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended
	Mar. 31, 2015	Mar. 31, 2014
Options to purchase common stock
Number of Shares
Outstanding at the beginning of the period (in shares)	19,957
Granted (in shares)	2,542
Exercised (in shares)	(1,240)
Cancelled (in shares)	(169)
Outstanding at the end of the period (in shares)	21,090
Weighted-Average Exercise Price
Outstanding at the beginning of the period (in dollars per share)	$ 10.07
Granted (in dollars per share)	$ 15.61
Exercised (in dollars per share)	$ 5.95
Cancelled (in dollars per share)	$ 13.01
Outstanding at the end of the period (in dollars per share)	$ 10.96
Weighted-average assumptions used to estimate the fair value of the stock options using the Black-Scholes option-pricing model
Expected volatility (as a percent)	46.00%	46.60%
Expected term	6 years	6 years 1 month 6 days
Risk-free interest rate (as a percent)	1.70%	1.80%