Fair Value Measurements - Additional Information (Detail) - USD ($)	3 Months Ended		9 Months Ended		12 Months Ended
	Feb. 28, 2018	Feb. 28, 2017	Feb. 28, 2018	Feb. 28, 2017	May 31, 2017
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Interest income	$ 12,000,000	$ 8,000,000	$ 36,000,000	$ 17,000,000
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	1,057,000,000		1,057,000,000
Available-for-sale securities with maturity dates over one year and less than five years from purchase date	32,000,000		32,000,000
Fair Value Measurements Using Level 2
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Fair value of long term debt	$ 3,327,000,000		$ 3,327,000,000		$ 3,401,000,000