Loans - Outstanding Pledged as Collateral (Details) - USD ($) $ in Thousands	Nov. 30, 2021	May 31, 2021
Pledging of Loans and Loans on Deposit
Cash	$ 11,010	$ 8,298
Collateral trust bonds 2007 indenture
Pledging of Loans and Loans on Deposit
Loans outstanding and pledged as collateral	8,295,416	8,521,972
Secured debt	7,022,711	7,422,711
Collateral trust bonds 1994 indenture
Pledging of Loans and Loans on Deposit
Secured debt	25,000	30,000
Notes payable | Federal financing bank
Pledging of Loans and Loans on Deposit
Secured debt	6,290,600	6,269,303
Notes payable | Federal agricultural mortgage corporation
Pledging of Loans and Loans on Deposit
Secured debt	3,121,485	2,977,909
Clean renewable energy bonds series 2009A
Pledging of Loans and Loans on Deposit
Secured debt	4,412	4,412
Cash	1,179	394
Assets pledged as collateral	5,699	5,710
Mortgage notes | Distribution system mortgage notes | Collateral trust bonds 2007 indenture
Pledging of Loans and Loans on Deposit
Loans outstanding and pledged as collateral	8,160,235	8,400,293
Mortgage notes | Distribution system mortgage notes | Collateral trust bonds 1994 indenture
Pledging of Loans and Loans on Deposit
Loans outstanding and pledged as collateral	32,241	34,924
Mortgage notes | Distribution and power supply system mortgage notes | Federal agricultural mortgage corporation
Pledging of Loans and Loans on Deposit
Loans outstanding and pledged as collateral	3,531,936	3,440,307
Mortgage notes | Distribution and power supply system mortgage notes | Clean renewable energy bonds series 2009A
Pledging of Loans and Loans on Deposit
Loans outstanding and pledged as collateral	4,520	5,316
Loans guaranteed by rural utilities service | Collateral trust bonds 2007 indenture
Pledging of Loans and Loans on Deposit
Loans outstanding and pledged as collateral	135,181	121,679
Mortgage notes receivable on deposit | Distribution and power supply system mortgage notes | Federal financing bank
Pledging of Loans and Loans on Deposit
Loans outstanding and pledged as collateral	$ 7,032,830	$ 7,150,240