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Stock Based Compensation (Details 4) - USD ($)
$ in Millions
12 Months Ended
May. 31, 2015
May. 31, 2014
May. 31, 2013
Ratio Of Outstanding And Available To Grant Shares To Total Outstanding Common And Equity Compensation Shares And Equity Compensation Shares Available For Grant [Abstract]      
Ratio Of Outstanding And Available To Grant Shares To Total Outstanding Common And Equity Compensation Shares And Equity Compensation Shares Available For Grant 9.00%    
Share Based Compensation Arrangement By Share Based Award Options Vesting During Years [Abstract]      
Vested during the year 2,611,524 2,408,179 2,824,757
Fair Value $ 83 $ 65 $ 81
Unrecognized Compensation Cost Related To Unvested Share Based Compensation Arrangements [Abstract]      
Total unrecognized compensation cost, net of estimated forfeitures $ 183    
Stock option remaining weighted average vesting period