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Stock Based Compensation (Tables)
 12 Months Ended
Dec. 31, 2017
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]  
Costs Related to Company's Stock-Based Compensation Plan
The following costs related to the Company’s stock-based compensation plan are included in the consolidated statements of income:
 
Year ended December 31,
 
2017
 
2016
 
2015
Cost of revenue
$
4,600

 
$
3,664

 
$
2,895

General and administrative expenses
10,363

 
8,372

 
6,077

Selling and marketing expenses
8,078

 
7,734

 
7,075

Total
$
23,041

 
$
19,770

 
$
16,047
Stock Based Compensation Stock Option Activity
Stock option activity under the Company’s stock plans is shown below:

Number of
Options
 
Weighted-
Average
Exercise
Price
 
Aggregate
Intrinsic
Value
 
Weighted-
Average
Remaining
Contractual
Life (Years)
Outstanding at December 31, 2016
811,902

 
$
16.31

 
$
27,718


2.96
  Granted
—

 
—

 

 

  Exercised
(552,339
)
 
15.50

 

 

  Forfeited
—

 
—

 

 

Outstanding at December 31, 2017
259,563

 
$
18.03

 
$
10,985

 
2.76
Vested and exercisable at December 31, 2017
259,563

 
$
18.03

 
$
10,985

 
2.76
Company's Stock Options Outstanding and Stock Options Vested and Exercisable
The following table summarizes the status of the Company’s stock options outstanding, vested and exercisable at December 31, 2017:
 
Options Outstanding, Vested and Exercisable
Range of Exercise Prices
Shares
 
Weighted-
Average
Exercise Price
$8.00 to $15.00
98,784

 
$
9.64

$15.01 to $21.00
36,516

 
19.05

$21.01 to $28.00
124,263

 
24.40

Total
259,563

 
$
18.03
Weighted Average Valuation Assumptions for Market Condition Performance Restricted Stock Units
The fair value of each MU granted to employees is estimated on the date of grant using the following weighted average assumptions:
 
Year ended December 31,
 
2017
 
2016
 
2015
Dividend yield
—

 
—

 
—

Expected life (years)
2.86

 
2.85

 
2.84

Risk free interest rate
1.40
%
 
0.88
%
 
0.98
%
Volatility
23.8
%
 
28.0
%
 
30.3
%
Restricted Stock and Restricted Stock Units  
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]  
Restricted Stock Activity Under Company's Stock Plans
Restricted stock and restricted stock unit activity under the Company’s stock plans is shown below:
 
Restricted Stock
 
Restricted Stock Units
 
Number
 
Weighted-
Average
Fair Value
 
Number
 
Weighted-
Average
Fair Value
December 31, 2016 (1)
246,940

 
$
42.42

 
1,256,288

 
$
37.38

  Granted
—

 
—

 
391,927

 
48.02

  Vested
(60,168
)
 
42.30

 
(489,176
)
 
34.52

  Forfeited
(4,505
)
 
35.11

 
(112,040
)
 
41.50

December 31, 2017 (1)
182,267

 
$
42.64

 
1,046,999

 
$
42.26

 
 
 
 
 
(1)     Excludes 11,058 and 19,874 restricted stock units vested during the years ended December 31, 2017 and 2016, respectively. As of December 31, 2017 and 2016 restricted stock units vested for which the underlying common stock is yet to be issued are 146,112 and 135,054, respectively.
Performance Based Stock Awards  
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]  
Restricted Stock Activity Under Company's Stock Plans
Performance restricted stock unit activity under the Company’s stock plans is shown below:
 
Revenue Based PRSUs
 
Market Condition Based PRSUs
 
Number
 
Weighted Avg
Fair Value
 
Number
 
Weighted Avg
Fair Value
Outstanding at December 31, 2016
115,174

 
$
41.70

 
215,171

 
$
47.42

Granted
62,113

 
47.73

 
62,100

 
54.10

Adjustment upon final determination of level of performance goal achievement*
(9,320
)
 
34.75

 
154,513

 
39.60

Vested
(45,192
)
 
34.75

 
(309,026
)
 
39.60

Forfeited
(9,585
)
 
44.35

 
(9,584
)
 
58.85

Outstanding at December 31, 2017
113,190

 
$
48.13

 
113,174

 
$
60.80


* Represents adjustment of shares issued in respect of PUs and MUs granted in February 2015 and MUs granted in April 2015 upon certification of the level of achievement of the performance targets for such awards for which the underlying common stock was issued subsequent to December 31, 2017.