Short-Term Debt and Credit Arrangements (Details) (USD $) In Thousands, unless otherwise specified	Aug. 31, 2013	May 31, 2013
Short-Term Debt Outstanding
Total short-term debt	$ 6,902,725	$ 7,719,483
Short-term debt
Short-Term Debt Outstanding
Short-term debt	4,450,315	4,050,132
Commercial paper sold through dealers, net of discounts
Short-Term Debt Outstanding
Short-term debt	2,289,831	2,009,884
Commercial paper sold directly to members, at par
Short-Term Debt Outstanding
Short-term debt	919,625	812,141
Commercial paper sold directly to non-members, at par
Short-Term Debt Outstanding
Short-term debt	20,954	39,298
Select notes
Short-Term Debt Outstanding
Short-term debt	493,968	358,390
Daily liquidity fund notes sold directly to members
Short-Term Debt Outstanding
Short-term debt	575,937	680,419
Bank bid notes
Short-Term Debt Outstanding
Short-term debt	150,000	150,000
Long-term debt maturing within one year
Short-Term Debt Outstanding
Total long-term debt maturing within one year	2,452,410	3,669,351
Medium-term notes sold through dealers
Short-Term Debt Outstanding
Total long-term debt maturing within one year	630,480	989,607
Medium-term notes sold to members
Short-Term Debt Outstanding
Total long-term debt maturing within one year	423,106	391,318
Secured collateral trust bonds
Short-Term Debt Outstanding
Total long-term debt maturing within one year	904,982	1,504,949
Member subordinated certificates
Short-Term Debt Outstanding
Total long-term debt maturing within one year	37,707	37,176
Secured notes payable
Short-Term Debt Outstanding
Total long-term debt maturing within one year	452,232	742,402
Unsecured notes payable
Short-Term Debt Outstanding
Total long-term debt maturing within one year	3,903	3,899
Short-term debt, including current portion of long-term debt
Short-Term Debt Outstanding
Total short-term debt	$ 6,902,725	$ 7,719,483