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Stock Based Compensation (Tables)
 12 Months Ended
Dec. 31, 2018
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]  
Share-based Compensation, Activity [Table Text Block]
Following is a summary of stock option activity for the year ended December 31, 2018, relating to the stock options awarded under the 2018 LTIP, the 2009 LTIP and the 1993 Plan:
 
Shares
 
Weighted
Average
Exercise
Price
 
Weighted Average Remaining Contractual Term (Years)
 
Aggregate Intrinsic Value(a)
(In thousands)
Options outstanding at December 31, 2017
1,822,818

 
$
20.48

 
6.88
 
$
25,376

Granted
438,500

 
31.89

 
 
 
 
Exercised
(38,628
)
 
13.92

 
 
 
 
Forfeited
(10,000
)
 
28.49

 
 
 
 
Options outstanding at December 31, 2018
2,212,690

 
$
22.82

 
6.58
 
$
3,123

Options vested or expected to vest at December 31, 2018
2,151,750

 
$
22.74

 
6.53
 
$
3,073

Options exercisable at December 31, 2018
1,400,490

 
$
21.22

 
5.65
 
$
2,486

(a)Intrinsic value is defined as the amount by which the fair value of the underlying common shares exceeds the exercise price of the option.
Five Year Service Stock Options [Member]  
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]  
Schedule of Share-based Payment Award, Stock Options, Valuation Assumptions [Table Text Block]
The fair value of our five-year service-based stock options granted during the years ended December 31, 2018, 2017 and 2016 was established at the date of grant using the Black-Scholes pricing model, with the weighted average assumptions as follows:
 
Year Ended December 31,
 
2018
 
2017
 
2016
Risk-free interest rate
2.72
%
 
1.96
%
 
1.34
%
Expected volatility
32.01
%
 
39.49
%
 
47.20
%
Expected term (in years)
5.7

 
5.9

 
5.7

Weighted average grant date fair value of options granted during the period
$
11.31

 
$
9.45

 
$
7.57