Long-Term Debt (Details) (USD $) In Thousands, unless otherwise specified	Aug. 31, 2013	May 31, 2013	Aug. 31, 2013 Long-Term Debt, excluding subordinated debt	May 31, 2013 Long-Term Debt, excluding subordinated debt	Aug. 31, 2013 Unsecured long-term debt	May 31, 2013 Unsecured long-term debt	Aug. 31, 2013 Unsecured medium-term notes	May 31, 2013 Unsecured medium-term notes	Aug. 31, 2013 Medium-term notes sold through dealers	May 31, 2013 Medium-term notes sold through dealers	Aug. 31, 2013 Medium-term notes sold to members	May 31, 2013 Medium-term notes sold to members	Aug. 31, 2013 Unsecured notes payable	May 31, 2013 Unsecured notes payable	Aug. 31, 2013 Secured long-term debt	May 31, 2013 Secured long-term debt	Aug. 31, 2013 Collateral trust bonds	May 31, 2013 Collateral trust bonds	Jun. 06, 2013 2.35 percent collateral trust bonds due 2020	Aug. 31, 2013 Secured notes payable	May 31, 2013 Secured notes payable
Long-term debt
Long-term debt before unamortized discount							$ 1,689,764	$ 1,711,214	$ 1,538,175	$ 1,528,424	$ 151,589	$ 182,790	$ 4,034,075	$ 3,709,074			$ 5,039,372	$ 4,639,372	$ 400,000
Stated interest rate (as a percent)																			2.35%
Unamortized discount							(624)	(627)					(883)	(920)			(180,952)	(181,640)
Total long-term debt	$ 11,684,259	$ 10,696,433	$ 11,384,259	$ 10,696,433	$ 5,722,332	$ 5,418,741	$ 1,689,140	$ 1,710,587					$ 4,033,192	$ 3,708,154	$ 5,961,927	$ 5,277,692	$ 4,858,420	$ 4,457,732		$ 1,103,507	$ 819,960