Summary of Significant Accounting Policies (Details) $ in Thousands	9 Months Ended
	Feb. 29, 2016 USD ($) subsidiary	May. 31, 2015 USD ($)	Feb. 28, 2015 USD ($)
General Information and Accounting Policies [Line Items]
Number of Groups of Subsidiaries Wholly Owned by Entity Holding Foreclosed Assets | subsidiary	1
Loans to members, net	$ 23,106,181	$ 21,435,327	$ 21,158,978
Unamortized debt issuance costs		$ 47,000
Consolidated variable interest entities | RTFC
General Information and Accounting Policies [Line Items]
Guarantee amount	2,000
Total assets including loans outstanding to members	464,000
Loans to members, net	358,000
Maximum amount committed to extend loan	4,000,000
Commitment outstanding	340,000
Consolidated variable interest entities | NCSC
General Information and Accounting Policies [Line Items]
Guarantee amount	49,000
Maximum potential exposure	53,000
Total assets including loans outstanding to members	711,000
Loans to members, net	697,000
Maximum amount committed to extend loan	3,000,000
Commitment outstanding	725,000
Loans outstanding	$ 676,000