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           Income Taxes (Details 2) 
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        12 Months Ended | ||
|---|---|---|---|
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           May 31, 2013 
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           May 31, 2012 
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           May 31, 2011 
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| Income Tax Disclosure [Abstract] | |||
| Federal statutory tax rate | 35.00% | 35.00% | 35.00% | 
| Increase/(decrease) resulting from: | |||
| State income taxes, net of federal tax benefit | 3.00% | 3.30% | 2.50% | 
| Tax settlement | 1.50% | 0.00% | 0.00% | 
| Tax-exempt municipal bond interest | (1.70%) | (1.80%) | (2.20%) | 
| Other items | (0.20%) | (0.20%) | (0.30%) | 
| Effective income tax rate | 37.60% | 36.30% | 35.00% | 
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- Definition 
          A ratio calculated by dividing the reported amount of income tax expense attributable to continuing operations for the period by GAAP-basis pretax income from continuing operations. Reference 1: http://www.xbrl.org/2003/role/presentationRef 
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- Definition 
          The domestic federal statutory tax rate applicable under enacted tax laws to the Company's pretax income from continuing operations for the period. The "statutory" tax rate is the regular tax rate if there are alternative tax systems. Reference 1: http://www.xbrl.org/2003/role/presentationRef 
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- Definition 
          The portion of the difference between the effective income tax rate and domestic federal statutory income tax rate attributable to all other items not otherwise listed in the existing taxonomy. Reference 1: http://www.xbrl.org/2003/role/presentationRef 
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- Definition 
          The portion of the difference between the effective income tax rate and domestic federal statutory income tax rate that can be explained by the state and local income tax expense or benefit, net of the federal tax benefit (expense) thereon, recorded during the period. Reference 1: http://www.xbrl.org/2003/role/presentationRef 
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- Definition 
          The portion of the difference between the effective income tax rate and domestic federal statutory income tax rate attributable to income that is exempt from income taxes under enacted tax laws. Reference 1: http://www.xbrl.org/2003/role/presentationRef 
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- Definition 
          The sum of the differences between the effective income tax rate and domestic federal statutory income tax rate attributable to all income tax settlements during the period. Reference 1: http://www.xbrl.org/2003/role/presentationRef 
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- Details 
          
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