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Stock-based compensation
 3 Months Ended
Mar. 31, 2018
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]  
Stock-based compensation
Stock-based compensation
Service-based vesting stock options
The following table summarizes the activity for options that vest solely based upon the satisfaction of a service condition for the three months ended March 31, 2018:
 
Options
outstanding
 
Weighted-average exercise price
 
Weighted-average remaining
contractual life
(in years)
 
Aggregate intrinsic
values
(in thousands)
Balance as of December 31, 2017
2,597,294

 
$
10.66

 
 
 
 

Granted
585,250

 
18.43

 
 
 
 

Exercised
(28,463
)
 
5.66

 
 
 
 

Forfeited
(72,505
)
 
6.45

 
 
 
 

Canceled
(30,032
)
 
4.60

 
 
 
 
Balance as of March 31, 2018
3,051,544

 
$
12.35

 
8.1 years
 
$
22,479

 
 
 
 
 
 
 
 
Exercisable, March 31, 2018
1,326,527

 
$
6.89

 
6.9 years
 
$
16,797


The aggregate intrinsic value is calculated as the difference between the exercise price of the underlying awards and the Company's closing stock of $19.37, as reported on the New York Stock Exchange on March 31, 2018.
The Company recognized stock-based compensation cost related to service-based vesting options of $0.7 million and $0.5 million in the three months ended March 31, 2018 and 2017, respectively. As of March 31, 2018, there was $10.2 million of total unrecognized stock-based compensation cost related to unvested service-based stock options, which is expected to be recognized over the remaining weighted-average period of 3.1 years. All stock-based compensation expense is recorded in selling, general and administrative expenses.
Performance-based and market-based vesting stock options
The following table summarizes the activity for options that vest based upon the satisfaction of performance or market conditions for the three months ended March 31, 2018:
 
Options
outstanding
 
Weighted-average exercise price
 
Weighted-average remaining
contractual life
(in years)
 
Aggregate intrinsic
values
(in thousands)
Balance as of December 31, 2017
2,406,537

 
$
6.58

 
 
 
 
Exercised
(23,460
)
 
2.16

 
 
 
 
Forfeited
(9,000
)
 
26.84

 
 
 
 
Canceled
(13,250
)
 
3.51

 
 
 
 
Balance as of March 31, 2018
2,360,827

 
$
6.56

 
7.2 years
 
$
33,334


The aggregate intrinsic value is calculated as the difference between the exercise price of the underlying awards and the Company's closing stock of $19.37, as reported on the New York Stock Exchange on March 31, 2018.
The Company recognized stock-based compensation cost related to performance-based and market-based vesting options of $0.4 million and $0.8 million in the three months ended March 31, 2018 and 2017, respectively. As of March 31, 2018, there was $0.7 million of total unrecognized stock-based compensation cost related to unvested market-based stock options, which is expected to be recognized over a weighted-average period of 0.4 years.
Restricted stock
The following table summarizes the activities for restricted stock awards (“RSAs”) and restricted stock units (“RSUs”) for the three months ended March 31, 2018:
 
Shares of restricted stock outstanding
 
Weighted-average grant date fair value
Balance as of December 31, 2017
1,248,824

 
$
23.37

Granted
1,012,990

 
18.43

Vested
(172,065
)
 
26.80

Forfeited
(29,320
)
 
22.76

Balance as of March 31, 2018
2,060,429

 
$
20.66


As of March 31, 2018, there were 885,520 unvested shares subject to RSAs outstanding.
The Company recognized stock-based compensation cost related to RSAs and RSUs of $2.5 million and $1.1 million in the three months ended March 31, 2018 and 2017, respectively. As of March 31, 2018, there was $39.4 million of total unrecognized stock-based compensation cost related to unvested RSAs and RSUs, which is expected to be recognized over a weighted-average period of 3.3 years.