Guarantees Guarantees Outstanding (Details) - USD ($) $ in Thousands		May 31, 2018	May 31, 2017
Guarantor Obligations [Line Items]
Guarantor Obligations, Current Carrying Value		$ 805,161	$ 889,617
Long-term tax-exempt bonds
Guarantor Obligations [Line Items]
Guarantor Obligations, Current Carrying Value	[1]	316,985	468,145
Letters of credit
Guarantor Obligations [Line Items]
Guarantor Obligations, Current Carrying Value	[2]	343,970	307,321
Other guarantees
Guarantor Obligations [Line Items]
Guarantor Obligations, Current Carrying Value		144,206	114,151
CFC
Guarantor Obligations [Line Items]
Guarantor Obligations, Current Carrying Value		793,156	874,920
CFC | Distribution
Guarantor Obligations [Line Items]
Guarantor Obligations, Current Carrying Value		201,993	126,188
CFC | Power supply
Guarantor Obligations [Line Items]
Guarantor Obligations, Current Carrying Value		587,837	743,678
CFC | Statewide and associate
Guarantor Obligations [Line Items]
Guarantor Obligations, Current Carrying Value		3,326	5,054
NCSC
Guarantor Obligations [Line Items]
Guarantor Obligations, Current Carrying Value		1,574	1,574
RTFC
Guarantor Obligations [Line Items]
Guarantor Obligations, Current Carrying Value		$ 10,431	$ 13,123

[1] Represents the outstanding principal amount of long-term fixed-rate and variable-rate guaranteed bonds.

[2] Reflects our maximum potential exposure for letters of credit.