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Stockholders' Equity (Deficit) (Tables)
 3 Months Ended
Mar. 31, 2015
Equity and Disclosure of Compensation Related Costs, Share-based Payments [Abstract]  
Schedule of Share-based Compensation, Stock Options, Activity
A summary of stock option activity for the three months ended March 31, 2015 is as follows (in thousands, except share and per share amounts): 
 
Options
Outstanding
 
Weighted-
Average
Exercise
Price
 
Aggregate
Intrinsic
Value
Balance at December 31, 2014
16,392,539

 
$
4.40

 
$
128,541

Stock option grants
466,900

 
$
8.88

 
 
Stock options exercised
(1,925,731
)
 
$
1.02

 
 
Stock options canceled
(1,016,745
)
 
$
7.12

 
 
Balance at March 31, 2015
13,916,963

 
$
4.82

 
$
61,110
Schedule of Share-based Payment Award, Stock Options, Valuation Assumptions
The fair value of each option grant was estimated on the date of grant using the Black-Scholes option-valuation model with the following assumptions and fair value per share:
 
Three Months Ended March 31,
 
2015
 
2014
Volatility
53%
 
60%
Expected life (in years)
6.2
 
5.0-6.2
Risk-free interest rate
1.38%-1.79%
 
1.53%-1.95%
Dividend yield
—%
 
—%