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Income Taxes (Details) (USD $)
In Thousands, unless otherwise specified
3 Months Ended 6 Months Ended
Feb. 28, 2013
Feb. 29, 2012
Feb. 28, 2013
Feb. 29, 2012
Income Taxes [Abstract]        
Income tax expense $ 10,137 $ 6,653 $ 17,792 $ 8,173
Estimated effective income tax rate     34.30% 34.20%