Stock-Based Compensation (Tables)
 9 Months Ended
Sep. 30, 2019
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]  
Schedule of Stock-based Compensation Expense
The following table summarizes stock-based compensation expense recognized for the periods presented:
 
For the Three Months Ended September 30,
 
For the Nine Months Ended September 30,
(in thousands)
2019
 
2018
 
2019
 
2018
Restricted stock awards
$
4,569

 
$
2,393

 
$
11,159

 
$
6,157

Stock option awards
2,557

 
2,337

 
7,766

 
6,853

Performance stock units
918

 
611

 
2,360

 
1,319

Other stock-based compensation expense(1)
66

 
—

 
66

 
—

Total stock-based compensation expense
$
8,110

 
$
5,341

 
$
21,351

 
$
14,329

 
(1)  
Includes expenses related to the Company’s Employees 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.
Schedule of Restricted Stock Activity
The following table provides information about restricted stock activity during the nine months ended September 30, 2019:
 
Awards
 
Weighted Average Grant Date Fair Value
Unvested balance as of December 31, 2018
1,535,945

 
$
17.88

Granted
3,906,196

 
6.70

Vested
(509,833
)
 
17.82

Forfeited
(45,217
)
 
12.96

Unvested balance as of September 30, 2019
4,887,091

 
8.99
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 options awarded during the nine months ended September 30, 2019 and 2018:

For the Nine Months Ended September 30,

2019

2018
Weighted average grant date fair value per share
$
4.47


$
7.74

Expected term (in years)
6


6

Expected stock volatility
46
%

41
%
Dividend yield
—
%

—
%
Risk-free interest rate
2.3
%

2.6
%
Summary of Stock Option Activity
The following table provides information about stock option awards outstanding during the nine months ended September 30, 2019:
 
Options
 
Weighted Average Exercise Price
 
Weighted Average Remaining Term
(in years)
 
Aggregate Intrinsic Value
(in thousands)
Outstanding as of December 31, 2018
4,559,334

 
$
16.55

 
 
 
 
Granted
326,000

 
9.56

 
 
 
 
Exercised
—

 
—

 
 
 

Forfeited
(65,336
)
 
16.90

 
 
 
 
Expired
(15,998
)
 
17.88

 
 
 
 
Outstanding as of September 30, 2019
4,804,000

 
16.07

 
7.5
 
$
—

Exercisable as of September 30, 2019
2,654,623

 
16.14

 
7.2
 
$
—
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 grant-date fair value of performance stock units awarded during the nine months ended September 30, 2019 and 2018:
 
For the Nine Months Ended September 30,
 
2019
 
2018
Weighted average grant-date fair value per share
$
6.68

 
$
22.35

Number of simulations
1,000,000

 
1,000,000

Expected stock volatility
52.3
%
 
40.2
%
Dividend yield
—
%
 
—
%
Risk-free interest rate
1.8
%
 
2.8
%
Share-based Compensation, Performance Shares Award Outstanding Activity [Table Text Block]
The following table provides information about performance stock units outstanding during the nine months ended September 30, 2019:
 
Awards
 
Weighted Average Grant Date Fair Value
Unvested balance as of December 31, 2018
386,459

 
$
21.94

Granted
486,213

 
6.68

Vested
—

 
—

Forfeited
—

 
—

Unvested balance as of September 30, 2019
872,672

 
13.44