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Stock Based Compensation (Tables)
3 Months Ended
Mar. 31, 2014
Notes to Financial Statements [Abstract]  
Stock Based Compensation Expense
 
Three months ended
March 31,
 
2014
2013
Stock based compensation
$  14,571
$  12,977
Tax effect recognized in net income
5,100
4,542
Windfall tax benefit, net
7,485
4,312
Weighted average assumptions used to value option grants
 
Three months ended March 31,
 
2014
 
2013
Risk-free interest rate (%)
1.62
 
0.80
Expected option life (years)
5.24
 
5.18
Expected volatility (%)
27.62
 
36.29
Expected dividend yield (%)
0.59
 
0.57