Fair Value Measurements - Additional Information (Detail) - USD ($)	3 Months Ended		9 Months Ended		12 Months Ended
	Feb. 28, 2017	Feb. 29, 2016	Feb. 28, 2017	Feb. 29, 2016	May 31, 2016
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Interest income	$ 8,000,000	$ 4,000,000	$ 17,000,000	$ 8,000,000
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	1,915,000,000		1,915,000,000
Available-for-sale securities with maturity dates over one year and less than five years from purchase date	$ 224,000,000		$ 224,000,000