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Stock-Based Compensation (Tables)
 3 Months Ended
Mar. 31, 2018
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]  
Schedule of stock-based compensation
Stock-based compensation for the three months ended March 31, 2018 and 2017 was as follows:
 
Three Months Ended March 31,
 
2018
 
2017
Stock-based compensation expense
$
4,494

 
$
3,398
Summary of stock option activity
 
Number of RSUs
 
Weighted-Average Grant Date Fair Value
RSUs outstanding at beginning of period
386

 
$
51.06

RSUs granted
160

 
$
69.32

RSUs vested
(155
)
 
$
52.59

RSUs forfeited
(55
)
 
$
47.51

RSUs outstanding at end of period
336

 
$
59.64


Changes in the outstanding stock option awards during the three months ended March 31, 2018 were as follows:
 
Number of Options
 
Weighted-Average Exercise Price per Share
Options outstanding at beginning of period
317

 
$
18.97

Options exercised
(17
)
 
$
18.36

Options expired
(1
)
 
$
12.19

Options outstanding at end of period
299

 
$
19.02