Loans - Schedule of Pledged Collateral Loans (Details) - USD ($) $ in Thousands	Feb. 28, 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	7,806,492	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	5,743,393	6,456,852
Notes Payable | Farmer Mac:
Pledging of Loans and Loans on Deposit
Notes payable outstanding	3,929,086	3,780,461
Mortgage notes | Distribution system mortgage notes pledged | Collateral Trust Bonds 2007 Indenture
Pledging of Loans and Loans on Deposit
Loans outstanding and pledged as collateral	7,709,091	8,107,921
Mortgage notes | Distribution system mortgage notes pledged | Collateral Trust Bonds 1994 Indenture
Pledging of Loans and Loans on Deposit
Loans outstanding and pledged as collateral	12,885	14,575
Mortgage notes | Distribution and power supply system mortgage notes pledged | Federal Financing Bank
Pledging of Loans and Loans on Deposit
Loans outstanding and pledged as collateral	7,392,048	7,640,203
Mortgage notes | Distribution and power supply system mortgage notes pledged | Farmer Mac:
Pledging of Loans and Loans on Deposit
Loans outstanding and pledged as collateral	4,892,969	4,648,691
RUS-guaranteed loans qualifying as permitted investments pledged | Collateral Trust Bonds 2007 Indenture
Pledging of Loans and Loans on Deposit
Loans outstanding and pledged as collateral	$ 97,401	$ 104,628