Stock-Based Compensation (Tables)
 12 Months Ended
Dec. 31, 2020
Share-based Payment Arrangement [Abstract]  
Schedule of Compensation Cost for Share-based Payment Arrangements, Allocation of Share-based Compensation Costs by Plan
The following table summarizes stock-based compensation expense recognized for the periods presented:
Year Ended December 31,
(in thousands)202020192018
Equity Awards
Restricted stock awards$15,355 $15,929 $9,185 
Stock option awards1,980 9,562 9,433 
Performance stock units3,312 3,374 2,052 
Other stock-based compensation expense(1)
319 132 — 
Total stock-based compensation expense - equity awards20,966 28,997 20,670 
Liability Awards
Restricted stock units1,788 — — 
Performance stock units1,814 — — 
Total stock-based compensation - liability awards3,602 — — 
Total stock-based compensation expense$24,568 $28,997 $20,670 
Nonvested Restricted Stock Shares Activity
The following table provides a summary of the restricted stock activity during the year ended December 31, 2020:
AwardsWtd. Avg. Grant-Date Fair Value
Unvested balance as of December 31, 20194,838,996 $8.51 
Granted10,245,500 1.12 
Vested(2,093,937)8.31 
Forfeited(896,836)5.85 
Unvested balance as of December 31, 202012,093,723 2.33 
Schedule of Share-based Payment Award, Stock Options, Valuation Assumptions
The following table summarizes the assumptions and related information used to determine the grant-date fair value of stock options awarded for the periods presented:
Years Ended December 31,
202020192018
Weighted average grant-date fair value per share$1.16 $4.32 $8.58 
Expected term (in years)666
Expected stock volatility86 %47 %42 %
Dividend yield— — — 
Risk-free interest rate1.0 %2.2 %2.7 %
Schedule of Share-based Compensation, Stock Options, Activity
The following table provides information about stock option awards outstanding during the year ended December 31, 2020:
OptionsWtd. Avg. Exercise PriceWtd. Avg. Remaining Term
(in years)		Aggregate Intrinsic Value
(in thousands)
Outstanding as of December 31, 20194,764,167 $15.99 
Granted124,000 2.13 
Exercised(366)0.25 $— 
Forfeited(130,424)13.12 
Expired(2,394,043)16.34 
Outstanding as of December 31, 20202,363,334 15.07 6.5$78 
Exercisable as of December 31, 20202,011,809 15.98 6.2$— 
Schedule of Share-based Payment Award, Equity Instruments Other Than Options, Valuation Assumptions
The following table summarizes the key assumptions and related information used to determine the grant-date fair value of performance stock units awarded during the periods presented:
20192018
Weighted average grant-date fair value per unit$6.68 $22.35 
Number of simulations1,000,000 1,000,000 
Expected stock volatility52.3 %40.2 %
Dividend yield— %— %
Risk-free interest rate1.8 %2.8 %
Share-based Compensation, Performance Shares Award Outstanding Activity
The following table provides information about performance stock units outstanding during the year ended December 31, 2020:
AwardsWtd. Avg. Grant Date Fair Value
Unvested balance as of December 31, 2019872,672 $13.44 
Granted— — 
Vested— — 
Cancelled(193,391)21.53 
Forfeited— — 
Unvested balance as of December 31, 2020679,281 11.13 
Schedule of Share-based payment award, Equity instruments Other than Options, Valuation Assumptions
The following table summarizes the key assumptions and related information used to determine the fair value of the liability awards as of December 31, 2020:
Restricted stock unitsPerformance stock units
Number of simulations10,000,000 10,000,000 
Expected stock volatility123.2 %126.9 %
Dividend yield— %— %
Risk-free interest rate0.2 %0.1 %
Shares outstanding5,442,681 5,464,433