Loans - Nonperforming Loans (Details) $ in Thousands	3 Months Ended		9 Months Ended
	Nov. 30, 2021 borrower	Aug. 31, 2021 borrower	Feb. 28, 2022 USD ($) borrower	May 31, 2021 USD ($)	Feb. 28, 2021 USD ($)
Accounts, Notes, Loans and Financing Receivable [Line Items]
Outstanding Amount			$ 29,508,363	$ 28,415,107	$ 28,315,671
% of Total Loans Outstanding			100.00%	100.00%
Nonperforming financial instruments
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of Borrowers | borrower		4	2
Outstanding Amount			$ 207,254	$ 237,497
% of Total Loans Outstanding			0.70%	0.84%
Parent Company
Accounts, Notes, Loans and Financing Receivable [Line Items]
Outstanding Amount			$ 28,320,476	$ 27,287,856
% of Total Loans Outstanding			96.00%	96.00%
Parent Company | Brazos electric power cooperative
Accounts, Notes, Loans and Financing Receivable [Line Items]
Letters of credit outstanding, amount				$ 1,000
Parent Company | Power supply
Accounts, Notes, Loans and Financing Receivable [Line Items]
Outstanding Amount			$ 4,992,073	$ 5,154,312
% of Total Loans Outstanding			17.00%	18.00%
Parent Company | Nonperforming financial instruments | Power supply
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of Borrowers | borrower		2	2
Outstanding Amount			$ 207,254	$ 228,312
% of Total Loans Outstanding			0.70%	0.81%
RTFC
Accounts, Notes, Loans and Financing Receivable [Line Items]
Outstanding Amount			$ 457,740	$ 420,383
% of Total Loans Outstanding			2.00%	1.00%
RTFC | Nonperforming financial instruments
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of Borrowers | borrower	2	2	0
Outstanding Amount			$ 0	$ 9,185
% of Total Loans Outstanding			0.00%	0.03%