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Stock-Based Compensation (Tables)
 3 Months Ended
Aug. 31, 2015
Share-based Compensation [Abstract]  
Schedule of Compensation Cost for Share-based Payment Arrangements, Allocation of Share-based Compensation Costs by Plan
The following table summarizes the Company’s total stock-based compensation expense recognized in Operating overhead expense: 
 
 
Three Months Ended August 31,
(In millions)
 
2015
 
2014
Stock options(1)
 
$
39

 
$
30

ESPPs
 
7

 
6

Restricted stock
 
8

 
7

TOTAL STOCK-BASED COMPENSATION EXPENSE
 
$
54

 
$
43

(1)
Expense for stock options includes the expense associated with stock appreciation rights. Accelerated stock option expense is recorded for employees eligible for accelerated stock option vesting upon retirement. Accelerated stock option expense for the three month periods ended August 31, 2015 and 2014 was $6 million and $4 million, respectively.
Schedule of Share-based Payment Award, Stock Options, Valuation Assumptions
The weighted average assumptions used to estimate these fair values are as follows:
 
 
Three Months Ended August 31,
  
 
2015
 
2014
Dividend yield
 
1.0
%
 
1.2
%
Expected volatility
 
23.6
%
 
23.6
%
Weighted average expected life (in years)
 
5.8

 
5.8

Risk-free interest rate
 
1.7
%
 
1.7
%