Guarantees (Details) - USD ($) $ in Thousands	9 Months Ended
	Feb. 29, 2016	May. 31, 2015
Guarantees
Guarantee Liability Recorded	$ 18,000	$ 20,000
Guaranty Liabilities Contingent	1,000	1,000
Total	915,359	986,500
Amount of unsecured guarantees	$ 320,000	$ 434,000
Unsecured guarantees as a percentage of total guarantees (percent)	35.00%	44.00%
Long-term variable-rate bonds
Guarantees
Value of bonds issued under liquidity provider obligations	$ 483,000
CFC
Guarantees
Total	895,116	$ 952,875
CFC | Distribution
Guarantees
Total	132,391	172,104
CFC | Power supply
Guarantees
Total	745,640	763,746
CFC | Statewide and associate
Guarantees
Total	17,085	17,025
RTFC
Guarantees
Total	1,574	1,574
NCSC
Guarantees
Total	18,669	32,051
Financial guarantees | Long-term tax-exempt bonds
Guarantees
Total	476,860	489,520
Financial guarantees | Long-term fixed-rate bonds
Guarantees
Total	70,000
Maximum potential exposure	99,000
Financial guarantees | Long-term variable-rate bonds
Guarantees
Total	407,000	418,000
Letters of credit | Master letter of credit
Guarantees
Maximum additional amount potentially required to be issued	85,000
Letters of credit | Letters of credit
Guarantees
Total	324,710	382,233
Letters of credit | Letters of credit, secured
Guarantees
Amount of guarantee secured	119,000
Letters of credit | Adjustable and floating-rate tax-exempt bonds
Guarantees
Total	76,000
Other guarantees
Guarantees
Total	113,789	$ 114,747
Maximum potential exposure	$ 115,000