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Fair Value Measurements - Additional Information (Detail) - USD ($)
3 Months Ended 9 Months Ended 12 Months Ended
Feb. 28, 2019
Feb. 28, 2018
Feb. 28, 2019
Feb. 28, 2018
May 31, 2017
May 31, 2018
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]            
Interest income $ 20,000,000 $ 12,000,000 $ 60,000,000 $ 36,000,000    
Fair value transfers between fair value hierarchy levels     0   $ 0  
Available-for-sale Securities            
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]            
Available-for-sale securities with maturity dates within one year from purchase date 313,000,000   313,000,000      
Available-for-sale securities with maturity dates over one year and less than five years from purchase date 38,000,000   38,000,000      
Fair Value Measurements Using Level 2            
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]            
Fair value of long term debt $ 3,345,000,000   $ 3,345,000,000     $ 3,294,000,000