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Stock-Based Compensation
 6 Months Ended
Jun. 30, 2019
Share-based Payment Arrangement [Abstract]  
Stock-Based Compensation Stock-Based Compensation
Stock Options
A summary of stock option activity during the six months ended June 30, 2019 is as follows (in thousands, except for per share amounts):
 
Shares
 
Weighted-
Average
Exercise
Price (in
dollars per
share)
 
Weighted-
Average
Contractual
Life (in
years)
 
Aggregate
Intrinsic
Value
Options outstanding at December 31, 2018
5,463

 
$
36.88

 
 
 
 

Granted
925

 
19.34

 
 
 
 

Exercised
(341
)
 
8.40

 
 
 
 

Canceled
(454
)
 
34.23

 
 
 
 

Expired
(429
)
 
39.66

 
 
 
 

Options outstanding at June 30, 2019
5,164

 
$
35.63

 
7.13
 
$
9,593

Options exercisable at June 30, 2019
2,957

 
$
40.16

 
5.86
 
$
4,252



The weighted-average grant-date fair value per share of options granted during the three and six months ended June 30, 2019 was $14.26 and $12.68, respectively. As of June 30, 2019, there was approximately $33.3 million of total unrecognized compensation expense related to unvested stock options, which is expected to be recognized over a weighted-average period of approximately 2.58 years.
Restricted Stock Units
A summary of RSU activity during the six months ended June 30, 2019 is as follows (in thousands, except for per share amounts):
 
RSUs
 
Weighted-
Average
Grant Date
Fair Value 
(in dollars 
per share)
RSUs Outstanding at December 31, 2018
227

 
$
37.69

Granted
613

 
20.04

Vested
(75
)
 
38.59

Forfeited
(81
)
 
26.86

RSUs Outstanding at June 30, 2019
684

 
$
23.02


As of June 30, 2019, there was approximately $14.0 million of total unrecognized compensation expense related to unvested RSUs, which is expected to be recognized over a weighted-average period of approximately 2.94 years.
Performance Units
During the six months ended June 30, 2019, the Company awarded 79,000 performance restricted stock units (“PSUs”) to employees. Each PSU entitles the holder to receive one share of the Company’s common stock if and when the PSU vests. The PSUs vest upon achievement of certain performance targets within a pre-specified period from the grant date. The vesting of any earned units is subject to the employee’s continued service relationship with the Company through each vesting date.
A summary of PSU activity during the six months ended June 30, 2019 is as follows (in thousands, except for per share amounts):
 
PSUs
 
Weighted-
Average
Grant Date
Fair Value 
(in dollars 
per share)
PSUs Outstanding at December 31, 2018
—

 
$
—

Granted
79

 
19.20

Vested
—

 
—

Forfeited
—

 
—

PSUs Outstanding at June 30, 2019
79

 
$
19.20


As the performance condition must be met for the awards to vest, compensation cost will be recognized over the implicit service period and only if the performance condition is assessed as probable of achievement. As of June 30, 2019, the performance condition was not probable and, therefore, no expense recognized to date.
Employee Stock Purchase Plan
In September 2016, the Company initiated the first offering period under the Company’s 2016 Employee Stock Purchase Plan (the “ESPP”), pursuant to which eligible employees may purchase shares of the Company’s common stock on the last day of each predetermined six-month offering period at 85% of the lower of the fair market value per share at the beginning or end of the applicable offering period. The offering periods run from March 1 through August 31 and from September 1 through February 28 (or February 29, in a leap year) of each year.
As of June 30, 2019, the Company had recorded a liability of $0.6 million related to its ESPP obligations. In accordance with the terms of its ESPP, the Company recorded stock-based compensation expense of $0.3 million and $0.4 million for the three and six-month periods ended June 30, 2019 and 2018, respectively.