Loans - Internal Risk Rating (Details) - USD ($) $ in Thousands	May 31, 2021	May 31, 2020
Credit Quality
Total loans outstanding	$ 28,415,107	$ 26,690,854	[1]
CFC
Credit Quality
Total loans outstanding	27,287,856	25,607,657	[1]
CFC | Distribution
Credit Quality
Total loans outstanding	22,027,423	20,769,653	[1]
CFC | Power supply
Credit Quality
Total loans outstanding	5,154,312	4,731,506	[1]
CFC | Statewide and associate
Credit Quality
Total loans outstanding	106,121	106,498	[1]
NCSC
Credit Quality
Total loans outstanding	706,868	697,862	[1]
RTFC
Credit Quality
Total loans outstanding	420,383	385,335	[1]
Pass
Credit Quality
Total loans outstanding	27,529,242	26,319,975
Pass | CFC
Credit Quality
Total loans outstanding	26,415,768	25,250,606
Pass | CFC | Distribution
Credit Quality
Total loans outstanding	21,808,099	20,643,737
Pass | CFC | Power supply
Credit Quality
Total loans outstanding	4,517,408	4,516,595
Pass | CFC | Statewide and associate
Credit Quality
Total loans outstanding	90,261	90,274
Pass | NCSC
Credit Quality
Total loans outstanding	706,868	697,862
Pass | RTFC
Credit Quality
Total loans outstanding	406,606	371,507
Special Mention
Credit Quality
Total loans outstanding	269,387	32,703
Special Mention | CFC
Credit Quality
Total loans outstanding	264,795	23,967
Special Mention | CFC | Distribution
Credit Quality
Total loans outstanding	219,324	7,743
Special Mention | CFC | Power supply
Credit Quality
Total loans outstanding	29,611	0
Special Mention | CFC | Statewide and associate
Credit Quality
Total loans outstanding	15,860	16,224
Special Mention | RTFC
Credit Quality
Total loans outstanding	4,592	8,736
Substandard
Credit Quality
Total loans outstanding	378,981	170,468
Substandard | CFC
Credit Quality
Total loans outstanding	378,981	165,376
Substandard | CFC | Distribution
Credit Quality
Total loans outstanding	0	118,173
Substandard | CFC | Power supply
Credit Quality
Total loans outstanding	378,981	47,203
Substandard | RTFC
Credit Quality
Total loans outstanding	0	5,092
Doubtful [Member]
Credit Quality
Total loans outstanding	237,497	167,708
Doubtful [Member] | CFC
Credit Quality
Total loans outstanding	228,312	167,708
Doubtful [Member] | CFC | Power supply
Credit Quality
Total loans outstanding	228,312	167,708
Doubtful [Member] | RTFC
Credit Quality
Total loans outstanding	$ 9,185	$ 0

[1] The interest rate on unadvanced loan commitments is not set until an advance is made; therefore, all unadvanced long-term loan commitments are reported as variable rate. However, the borrower may select either a fixed or a variable rate when an advance is drawn under a loan commitment.(2) Represents the unpaid principal balance, net of charge-offs and recoveries, of loans as of the end of each period.