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Stock-based compensation (Tables)
 12 Months Ended
Dec. 31, 2018
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]  
Summary of Stock Option Activity and Related Information
 
Options
outstanding
 
Weighted-average exercise price 
 
Weighted-average remaining
contractual life
(in years)
 
Aggregate intrinsic
values
(in thousands) (1)
Balance as of December 31, 2017
2,406,537

 
$
6.58

 
 
 
 
Exercised
(901,505
)
 
2.54

 
 
 
 
Canceled or forfeited
(22,250
)
 
12.95

 
 
 
 
Balance as of December 31, 2018
1,482,782

 
$
8.94

 
6.3
 
$
7,120

 
 
 
 
 
 
 
 
Exercisable, December 31, 2018
1,068,482

 
$
2.00

 
5.7
 
$
7,120

(1)The aggregate intrinsic value is calculated as the difference between the exercise price of the underlying awards and the Company's closing stock price of $8.66, as reported on the New York Stock Exchange on December 31, 2018.

The following table summarizes the activity for options that vest solely based upon the satisfaction of a service condition as follows:
 
Options
outstanding
 
Weighted-average exercise price
 
Weighted-average remaining
contractual life
(in years)
 
Aggregate intrinsic
values
(in thousands) (1)
Balance as of December 31, 2017
2,597,294

 
$
10.66

 
 
 
 

Granted
585,250

 
18.43

 
 
 
 

Exercised
(236,490
)
 
3.75

 
 
 
 

Canceled or forfeited
(199,384
)
 
10.56

 
 
 
 

Balance as of December 31, 2018
2,746,670

 
$
12.91

 
7.6
 
$
4,880

 
 
 
 
 
 
 
 
Exercisable, December 31, 2018
1,399,451

 
$
9.14

 
6.9
 
$
4,200

(1)
The aggregate intrinsic value is calculated as the difference between the exercise price of the underlying awards and the Company's closing stock price of $8.66, as reported on the New York Stock Exchange on December 31, 2018.
Summary of Additional Information Relating to Stock Options Activity
Additional information relating to service-based options is as follows (in thousands, except per share data):
 
Year ended December 31,
 
2018
 
2017
 
2016
Stock-based compensation expense
$
3,219

 
$
2,435

 
$
4,286

Intrinsic value of options exercised
$
2,890

 
$
12,841

 
$
2,486

Weighted-average grant date fair value
   of options granted (per share)
$
6.81

 
$
9.51

 
$
5.07

Additional information relating to options that vest based upon the satisfaction of performance or market conditions is as follows (in thousands, except per share data):
 
Year ended December 31,
 
2018
 
2017
 
2016
Stock-based compensation expense
$
1,168

 
$
3,489

 
$
1,813

Intrinsic value of options exercised
$
8,669

 
$
42,874

 
$
—

Weighted-average grant date fair value
   of options granted (per share)
$
—

 
$
10.65

 
$
1.52
Summary of Weighted-Average Assumptions
The fair value of service-based stock options granted were calculated using the following weighted-average assumptions:
 
Year ended December 31,
 
2018
 
2017
 
2016
Expected term (in years)
6.3

 
6.2

 
5.9

Expected volatility
32.02
%
 
32.42
%
 
36.50
%
Risk-free interest rate
2.68
%
 
2.14
%
 
1.34
%
Expected dividend yield
—
%
 
—
%
 
—
%
Summary of Restricted Stock Unit Activity and Related Information
The following table summarizes the activities for restricted stock awards (“RSAs”) and restricted stock units (“RSUs”) as follows:
 
Shares of restricted stock outstanding
 
Weighted-average grant date fair value
Balance as of December 31, 2017
1,248,824

 
$
23.37

Granted
1,331,745

 
17.73

Vested
(376,130
)
 
22.96

Canceled or forfeited
(168,315
)
 
20.24

Balance as of December 31, 2018
2,036,124

 
$
20.01
Disclosure of Additional Information
Additional information relating to RSAs and RSUs is as follows (in thousands):
 
Year ended December 31,
 
2018
 
2017
 
2016
Stock-based compensation expense
$
12,434

 
$
7,550

 
$
700

Intrinsic value of RSUs released
$
6,280

 
$
3,398

 
$
—