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Stock-Based Compensation (Tables)
 12 Months Ended
Dec. 31, 2017
Stock-Based Compensation [Abstract]  
Stock Option Valuation Assumptions
The fair value of stock option awards was estimated using the Black-Scholes option-pricing model with the following assumptions and weighted-average fair values as follows:

  
December 31,
 
Stock Options:
 
2015
  
2016
  
2017
 
          
Weighted-average grant date fair value of grants
 
$
16.26
  
$
12.59
  
$
18.84
 
Risk-free interest rate(1) 
  
1.7%
 
  
1.4%
 
  
2.1%
 
Dividend yield(2) 
  
2.1%
 
  
2.3%
 
  
2.5%
 
Expected volatility(3) 
  
46.8%
 
  
47.9%
 
  
48.2%
 
Expected life in months(4) 
 
65 months
  
68 months
  
68 months
 


(1)
The risk-free interest rate is based upon the rate on a zero-coupon U.S. Treasury bill, for periods within the contractual life of the option, in effect at the time of the grant.

(2)
The dividend yield is based on the average of historical stock prices and actual dividends paid.

(3)
Expected volatility is based on the historical volatility of the Company's stock price, over a period similar to the expected life of the option.

(4)
The expected term of the option is based on the historical employee exercise behavior, the vesting terms of the respective option, and a contractual life of either seven or ten years.
Stock Options
Options under the plans as of December 31, 2017 and changes during the year ended December 31, 2017 were as follows:

  
Shares
(in thousands)
  
Weighted-average Exercise Price
  
Weighted- average Remaining Contractual Term
(in years)
  
Aggregate Intrinsic Value
(in thousands)
 
             
Options activity – service based
            
Outstanding at December 31, 2016 
  
2,207.0
  
$
39.93
       
Granted 
 
─
  
─
       
Exercised 
  
(324.4
)
  
34.71
       
Forfeited/cancelled/expired 
  
(489.7
)
  
43.85
       
Outstanding at December 31, 2017 
  
1,392.9
   
39.76
   
4.36
  $
41,756
 
Exercisable at December 31, 2017 
  
725.5
   
43.14
   
3.55
   
20,237
 
                 
Options activity – performance based
                
Outstanding at December 31, 2016 
  
3,725.9
  
$
58.23
         
Granted 
  
298.0
   
50.93
         
Exercised 
  
(853.0
)
  
30.86
         
Forfeited/cancelled/expired 
  
(869.4
)
  
70.17
         
Outstanding at December 31, 2017 
  
2,301.5
   
62.89
   
3.61
  $
26,206
 
Exercisable at December 31, 2017 
  
294.1
   
35.58
   
4.17
   
9,601
 
                 
Options activity – all options
                
Outstanding at December 31, 2016 
  
5,932.9
  
$
51.42
         
Granted 
  
298.0
   
50.93
         
Exercised 
  
(1,177.4
)
  
31.92
         
Forfeited/cancelled/expired 
  
(1,359.1
)
  
60.69
         
Outstanding at December 31, 2017 
  
3,694.4
   
54.17
   
3.90
  $
67,962
 
Exercisable at December 31, 2017 
  
1,019.6
   
40.96
   
3.73
   
29,838
 
Stock Options Exercised
Cash proceeds, tax benefits and intrinsic value related to total stock options exercised during 2015, 2016 and 2017, were as follows (U.S. dollars in thousands):

  
December 31,
 
  
2015
  
2016
  
2017
 
          
Cash proceeds from stock options exercised
 
$
13,041
  
$
15,707
  
$
26,980
 
Tax benefit realized for stock options exercised
  
4,451
   
3,840
   
6,457
 
Intrinsic value of stock options exercised
  
12,085
   
30,587
   
42,749
 
Nonvested Restricted Stock Awards
Nonvested restricted stock awards as of December 31, 2017 and changes during the year ended December 31, 2017 were as follows:

  
Number of Shares
(in thousands)
  
Weighted-average Grant Date Fair Value
 
       
Nonvested at December 31, 2016 
  
596.4
  $
53.62
 
         
Granted 
  
261.6
   
50.95
 
Vested 
  
(203.2
)
  
53.94
 
Forfeited
  
(91.9
)
  
60.30
 
         
Nonvested at December 31, 2017 
  
562.9
   
51.17