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Stock Compensation
 6 Months Ended
Jun. 30, 2020
Share-based Payment Arrangement [Abstract]  
Stock Compensation Stock Compensation
Restricted Stock Units (“RSUs”) Activity

A summary of unvested restricted stock unit activity for the six months ended June 30, 2020 is as follows:

Number of
Shares		Weighted-
Average
Grant Date Fair Value
Balance as of December 31, 201911,615,884  $2.44  
Granted12,299,276  $0.88  
Vested(2,032,476) $2.77  
Forfeited and canceled
(3,268,027) $2.51  
Balance as of June 30, 202018,614,657  $1.36  

As of June 30, 2020, there was a total of $22.5 million in unrecognized compensation cost related to restricted stock units, which is expected to be recognized over a weighted-average period of approximately 2.7 years.

The Company granted approximately 0.9 million performance stock units ("PSUs") during the third quarter of 2019. The number of shares that will eventually vest depends on achievement of certain performance targets, as determined by the compensation committee of the Company's board of directors. Once the performance is determined, the PSUs, if any, will vest, subject to recipients' continued service, on the later of (i) the attainment of the performance targets and (ii) a year after the grant date. The compensation expense associated with the PSUs is recognized using the accelerated method. For the three and six months ended June 30, 2020, the Company recognized compensation expense of approximately $0.6 million and $0.8 million, respectively, related to these performance awards.

Stock Option Activity

A summary of stock option activity for the six months ended June 30, 2020 is as follows: 
Options
Outstanding		Weighted-
Average
Exercise
Price		Aggregate
Intrinsic
Value (in thousands)
Balance as of December 31, 20197,207,733  $1.94  $412  
Granted749,111  $1.17  
Exercised(142,729) $1.08  
Forfeited and canceled(2,117,240) $1.71  
Balance as of June 30, 20205,696,875  $1.94  $2  

The total grant-date fair value of stock options granted during the six months ended June 30, 2020 and 2019 was $0.6 million and $0.4 million, respectively.
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:

 Six Months Ended June 30,
 20202019
Volatility73%57%
Expected life (in years)
6.11 - 6.12
6.06
Risk-free interest rate
0.84% - 1.47%
2.57%
Dividend yield—%—%

As of June 30, 2020, the Company had $2.6 million in unrecognized compensation cost related to non-vested stock options, which is expected to be recognized over a weighted-average period of approximately 3.2 years. 
Employee Stock Purchase Plan
The Company used the following Black-Scholes assumptions in estimating the fair value of the shares under the 2014 Employee Stock Purchase Plan (the “ESPP”):

Six Months Ended June 30, 2020
Volatility71%
Expected life equals length of offering period (in years)0.5
Risk-free interest rate0.95%
Dividend yield—%

Stock-based compensation expense related to the ESPP was immaterial for the three and six months ended June 30, 2020. As of June 30, 2020, the unrecognized stock-based compensation expense related to the ESPP was also immaterial, and is expected to be recognized over the remaining term of the offering period.