Loans - Schedule of pledged loans (Details) - USD ($) $ in Thousands	May 31, 2026	May 31, 2025
Collateral trust bonds 2007 indenture
Pledging of Loans and Loans on Deposit
Notes payable outstanding	$ 6,665,191	$ 7,072,711
Loans outstanding and pledged as collateral	7,692,299	8,212,549
Collateral trust bonds 1994 indenture
Pledging of Loans and Loans on Deposit
Notes payable outstanding	5,000	10,000
Notes payable | Federal Financing Bank
Pledging of Loans and Loans on Deposit
Notes payable outstanding	5,338,722	6,456,852
Notes payable | Federal Agricultural Mortgage Corporation
Pledging of Loans and Loans on Deposit
Notes payable outstanding	3,911,993	3,780,461
Mortgage notes | Distribution system mortgage notes | Collateral trust bonds 2007 indenture
Pledging of Loans and Loans on Deposit
Loans outstanding and pledged as collateral	7,597,147	8,107,921
Mortgage notes | Distribution system mortgage notes | Collateral trust bonds 1994 indenture
Pledging of Loans and Loans on Deposit
Loans outstanding and pledged as collateral	7,155	14,575
Mortgage notes | Distribution and power supply system mortgage notes | Federal Financing Bank
Pledging of Loans and Loans on Deposit
Loans outstanding and pledged as collateral	6,744,950	7,640,203
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	4,837,570	4,648,691
Loans guaranteed by rural utilities service | Collateral trust bonds 2007 indenture
Pledging of Loans and Loans on Deposit
Loans outstanding and pledged as collateral	$ 95,152	$ 104,628