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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]  
Summarized information about stock options outstanding
Summarized information about stock options outstanding at December 31, 2017 under the 2003 Plan and the 2012 Incentive Plan is as follows.
 
 
Number of
options
(in thousands)
 
Weighted
average
exercise price
Options outstanding at December 31, 2016
 
2,887

 
$
15.59

Options granted
 
1,034

 
$
27.09

Options exercised
 
(833
)
 
$
9.20

Options forfeited
 
(24
)
 
$
23.81

Options expired
 
—

 
$
—

Options outstanding at December 31, 2017
 
3,064

 
$
21.14
Summarized information about outstanding and exercisable stock option
 
 
Options outstanding at
December 31, 2017
 
Options exercisable at
December 31, 2017
Range of exercise prices
 
Shares
outstanding (in thousands)
 
Weighted
average
remaining
contractual
life
 
Weighted
average
exercise
price
 
Shares
exercisable (in thousands)
 
Weighted
average
exercise
price
$8.18 - $9.55
 
294

 
0.69
 
$
8.46

 
294

 
$
8.46

$13.22 - $17.80
 
625

 
3.10
 
$
15.01

 
11

 
$
16.45

$19.71 - $22.70
 
839

 
2.05
 
$
21.87

 
73

 
$
21.50

$23.40 - $27.33
 
1,306

 
4.33
 
$
26.47

 
141

 
$
24.21
Summary of the non-vested restricted stock and restricted stock units
A summary of the non-vested restricted stock and restricted stock units as of December 31, 2017 is presented below (in thousands, except fair value).
 
 
Restricted Stock
 
Restricted Stock Units
Non-vested restricted stock and
restricted stock units
 
Shares
 
Weighted
average
fair
value
 
Shares
 
Weighted
average
fair
value
Non-vested at December 31, 2016
 
1,039

 
$
18.23

 
82

 
$
21.32

Granted
 
531

 
$
26.25

 
113

 
$
23.77

Vested
 
(429
)
 
$
16.54

 
(124
)
 
$
22.38

Forfeited
 
(37
)
 
$
22.94

 
(6
)
 
23.45

Non-vested at December 31, 2017
 
1,104

 
$
22.59

 
65

 
$
23.36
2003 Stock and Incentive Plan [Member]  
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]  
Weighted average assumptions used to estimate fair value of stock options granted under the Stock and Incentive Plan
The fair value of the 75,000, 77,500 and 87,500 stock option awards outstanding under the 2003 Plan at December 31, 2017, 2016 and 2015, respectively, was estimated using the following weighted average assumptions.
 
 
2017
 
2016
 
2015
Stock option pricing model
 
Black Scholes Merton
 
Black Scholes Merton
 
Black Scholes Merton
Expected option life
 
2.14 years
 
3.14 years
 
0.39 years
Risk-free interest rate
 
1.98%
 
1.70%
 
0.64%
Volatility
 
43.60%
 
47.07%
 
91.98%
Dividend yield
 
—%
 
—%
 
—%
Estimated forfeiture rate
 
—%
 
—%
 
—%
2012 Incentive Plan [Member]  
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]  
Weighted average assumptions used to estimate fair value of stock options granted under the Stock and Incentive Plan
The weighted average grant date fair value for stock option awards granted under the 2012 Incentive Plan was estimated using the following weighted average assumptions during the years ended December 31, 2017, 2016 and 2015.
 
 
2017
 
2016
 
2015
Stock option pricing model
 
Black Scholes Merton
 
Black Scholes Merton
 
Black Scholes Merton
Expected option life
 
4.00 years
 
3.96 years
 
4.00 years
Risk-free interest rate
 
1.77%
 
1.08%
 
1.15%
Volatility
 
47.00%
 
45.68%
 
56.89%
Dividend yield
 
—%
 
—%
 
—%
Estimated forfeiture rate
 
3.66%
 
1.16%
 
3.21%
Weighted average fair value of stock option awards granted during the year
 
$10.49
 
$5.65
 
$9.90