Stockholders' Equity (Deficit) (Tables)
 12 Months Ended
Dec. 31, 2015
Equity and Disclosure of Compensation Related Costs, Share-based Payments [Abstract]  
Share-based Compensation Arrangement by Share-based Payment Award, Options, Grants in Period, Weighted Average Grant Date Fair Value
 
Options Outstanding
 
Number of
Shares
Outstanding
 
Weighted-
Average
Exercise
Price
 
Weighted-
Average
Remaining
Contractual
Life in
Years
 
Aggregate
Intrinsic
Value
Balance at December 31, 2014
16,392,539

 
$
4.40

 
7.3
 
$
128,541

Stock options granted
491,900

 
$
8.95

 
 
 
 
Stock options exercised
(4,177,555
)
 
$
1.02

 
 
 
 
Stock options canceled and forfeited
(3,145,171
)
 
$
5.87

 
 
 
 
Balance at December 31, 2015
9,561,713

 
$
5.62

 
6.9
 
$
16,694

Vested or expected to vest December 31, 2015
9,022,878

 
$
5.50

 
6.8
 
$
16,149

Exercisable as of December 31, 2015
5,416,829

 
$
3.99

 
6.0
 
$
12,497
Nonvested Restricted Stock Shares Activity
The following table summarizes activities for RSUs:
 
Restricted Stock Units Outstanding
 
Number of shares
 
Weighted Average Grant-Date Fair Value
Balance at December 31, 2014
1,398,893

 
$
11.06

Restricted stock units granted
6,507,250

 
$
7.24

Restricted stock units vested
(295,468
)
 
$
10.70

Restricted stock units canceled and forfeited
(925,557
)
 
$
9.06

Balance at December 31, 2015
6,685,118

 
$
7.63
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:
 
Year Ended December 31,
 
2015
 
2014
 
2013
Volatility
53%
 
60%
 
57.8%-60%
Expected life (in years)
6.2
 
5.0-6.3
 
5.0-7.2
Risk-free interest rate
1.38%-1.91%
 
1.53%-2.05%
 
0.7%-1.8%
Dividend yield
—%
 
—%
 
—%
Weighted-average fair value of underlying common stock
$8.95
 
$14.74
 
$3.02