Guarantees - Narrative (Details) - USD ($)	12 Months Ended
	May 31, 2023	May 31, 2022	May 31, 2021
Guarantor Obligations [Line Items]
CFC third-party guarantees	$ 796,821,000	$ 730,783,000
Guarantee obligations unsecured	$ 535,000,000	$ 466,000,000
Guarantee obligations unsecured commitment as percentage of total commitment	67.00%	64.00%
Guarantee liability recorded	$ 13,000,000	$ 13,000,000
Guaranty liabilities		12,000,000
Other guarantees
Guarantor Obligations [Line Items]
CFC third-party guarantees	160,023,000	158,279,000
Guarantor obligations, maximum exposure, undiscounted	160,000,000	158,000,000
Financial standby letter of credit | Adjustable and floating rate tax exempt bonds
Guarantor Obligations [Line Items]
CFC third-party guarantees	0	0	$ 0
Performance guarantee | Master letter of credit
Guarantor Obligations [Line Items]
Letter of credit facility maximum additional amount potentially required to be issued	126,000,000
Letters of credit
Guarantor Obligations [Line Items]
CFC third-party guarantees	538,393,000	450,354,000
Guarantee obligations secured	138,000,000	118,000,000
Long-term tax-exempt bonds
Guarantor Obligations [Line Items]
CFC third-party guarantees	98,405,000	122,150,000
Guarantee obligations liquidity provided to member carrying value	98,000,000
Guarantee type, other secured
Guarantor Obligations [Line Items]
Guarantor obligations, maximum exposure, undiscounted	25,000,000	25,000,000
CFC
Guarantor Obligations [Line Items]
CFC third-party guarantees	$ 781,558,000	$ 706,813,000