Members' Subordinated Certificates (Details) - USD ($) $ in Millions	12 Months Ended
	May. 31, 2015	May. 31, 2014
Membership subordinated certificates
Members' subordinated certificates
Maturity period	100 years
Interest rate (as a percent)	5.00%
Membership subordinated certificates | Weighted-Average
Members' subordinated certificates
Maturity period	61 years	62 years
Series 2008 Member Capital Securities [Member]
Members' subordinated certificates
Maturity period	35 years
Interest rate (as a percent)	7.50%
Member capital security, call option term	5 years
Subordinated borrowings redeemed	$ 387
Member capital securities
Members' subordinated certificates
Maturity period	30 years
Interest rate (as a percent)	5.00%
Series 2013 Member Capital Securities [Member]
Members' subordinated certificates
Member capital security, call option term	10 years
Subordinated borrowings, current period investment	$ 219