Summary of Significant Accounting Policies (Details) (USD $)	Aug. 31, 2014	May 31, 2014	Aug. 31, 2013
General Information and Accounting Policies [Line Items]
Loans to members, net	$ 20,434,867,000	$ 20,420,213,000	$ 20,359,137,000
Consolidated variable interest entities | RTFC
General Information and Accounting Policies [Line Items]
Guarantee amount	2,000,000
Total assets including loans outstanding to members	558,000,000
Loans to members, net	437,000,000
Maximum amount committed to extend loan	4,000,000,000
Commitment outstanding	425,000,000
Consolidated variable interest entities | NCSC
General Information and Accounting Policies [Line Items]
Guarantee amount	85,000,000
Maximum potential exposure	91,000,000
Total assets including loans outstanding to members	663,000,000
Loans to members, net	639,000,000
Maximum amount committed to extend loan	3,000,000,000
Commitment outstanding	709,000,000
Loans outstanding	$ 624,000,000