Stock Based Compensation (Tables)

9 Months Ended

Sep. 30, 2012

Notes to Financial Statements [Abstract]

Stock Based Compensation Expense

	Three months ended September 30,				Nine months endedSeptember 30,
	2012		2011		2012		2011
Stock based compensation	$	10,439	$	7,660	$	30,143	$	23,466
Tax effect recognized in net income		3,654		2,681		10,550		8,216
Windfall tax benefit, net		9,009		372		19,975		3,999

Weighted average assumptions used to value option grants

	Nine months ended September 30,
	2012	2011
Fair value per share ($)	30.19	24.45
Risk-free interest rate (%)	0.77	1.91
Expected option life (years)	5.24	5.34
Expected volatility (%)	36.51	35.26
Expected dividend yield (%)	0.58	0.60