Stock Based Compensation (Tables)

6 Months Ended

Jun. 30, 2013

Notes to Financial Statements [Abstract]

Stock Based Compensation Expense

	Three months endedJune 30,				Six months endedJune 30,
	2013		2012		2013		2012
Stock based compensation	$	13,307	$	9,749	$	26,284	$	19,704
Tax effect recognized in net income		4,658		3,412		9,199		6,896
Windfall tax benefit/(shortfall), net		1,245		3,551		5,557		10,966

Weighted average assumptions used to value option grants

	Six months ended June 30,
	2013	2012
Fair value per share ($)	36.55	29.39
Risk-free interest rate (%)	0.78	0.82
Expected option life (years)	5.19	5.22
Expected volatility (%)	36.22	36.55
Expected dividend yield (%)	0.57	0.59