Stock Based Compensation (Tables)

6 Months Ended

12 Months Ended

Jun. 30, 2012

Dec. 31, 2011

Notes to Financial Statements [Abstract]

Stock Based Compensation Expense

	Three months ended June 30,				Six months ended June 30,
	2012		2011		2012		2011
Stock based compensation	$	9,749	$	7,704	$	19,704	$	15,808
Tax effect recognized in net income		3,412		2,696		6,896		5,535
Windfall tax benefit/(shortfall), net		3,551		836		10,966		3,627

Weighted average assumptions used to value option grants

	Six months ended June 30,
	2012	2011
Fair value per share ($)	29.39	24.86
Risk-free interest rate (%)	0.82	2.07
Expected option life (years)	5.22	5.33
Expected volatility (%)	36.55	35.17
Expected dividend yield (%)	0.59	0.59