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Stock-Based Compensation (Tables)
 6 Months Ended
Jun. 30, 2021
Share-based Payment Arrangement [Abstract]  
Schedule of Share-based payment award, Equity instruments Other than Options, Valuation Assumptions [Table Text Block]
The following table summarizes the key assumptions and related information used to determine the fair value of the liability awards as of June 30, 2021:
Restricted stock unitsPerformance stock units
Number of simulations10,000,00010,000,000
Expected implied stock volatility69.8%72.3%
Dividend yield—%—%
Risk-free interest rate0.3%0.3%
Schedule of Stock-based Compensation Expense
The following table summarizes stock-based compensation expense recognized for the periods presented:
Three Months Ended June 30,Six Months Ended June 30,
(in thousands)2021202020212020
Equity Awards
Restricted stock awards$3,536 $3,387 $7,142 $7,741 
Stock option awards234 291 505 1,275 
Performance stock units646 1,003 1,285 2,006 
Other stock-based compensation expense(1)
65 46 134 114 
Total stock-based compensation - equity awards4,481 4,727 9,066 11,136 
Liability Awards
Restricted stock units4,647 — 7,955 — 
Performance stock units10,013 — 17,119 — 
Total stock-based compensation - liability awards14,660 — 25,074 — 
Total stock-based compensation expense$19,141 $4,727 $34,140 $11,136 
(1)     Includes expenses related to the Company’s Employee Stock Purchase Plan (the “ESPP”). In May 2019, an aggregate of 2,000,000 shares were authorized by stockholders for issuance under the ESPP, which became effective on July 1, 2019. As of June 30, 2021, the Company had 1,561,164 shares of Common Stock available for future issuance.
Schedule of Restricted Stock Activity
The following table provides information about restricted stock activity during the six months ended June 30, 2021:
AwardsWeighted Average Grant-Date Fair Value
Unvested balance as of December 31, 202012,093,723 $2.33 
Granted— — 
Vested(504,052)5.39 
Forfeited(8,228)8.57 
Unvested balance as of June 30, 202111,581,443 2.19 
Summary of Stock Option Valuation Inputs
The following table summarizes the assumptions and related information used to determine the grant-date fair value of stock option awards for the six months ended June 30, 2020. No stock options were granted during the six months ended June 30, 2021.
Six Months Ended June 30,
20212020
Weighted average grant-date fair value per share$— $1.16 
Expected term (in years)06
Expected stock volatility— %86 %
Dividend yield— %— %
Risk-free interest rate— %1.0 %
Summary of Stock Option Activity
The following table provides information about stock option awards outstanding during the six months ended June 30, 2021:
OptionsWeighted Average Exercise PriceWeighted Average Remaining Term
(in years)		Aggregate Intrinsic Value
(in thousands)
Outstanding as of December 31, 20202,363,334 $15.07 
Granted— — 
Exercised(14,999)0.99 $71 
Forfeited(5,334)5.77 
Expired(52,333)16.98 
Outstanding as of June 30, 20212,290,668 15.14 6.03$473 
Exercisable as of June 30, 20212,057,977 15.73 5.82$103