Guarantees - Guarantees Outstanding (Details) - USD ($)	Aug. 31, 2023	May 31, 2023
Guarantees
Guarantor obligations	$ 900,441,000	$ 796,821,000
Parent Company
Guarantees
Guarantor obligations	883,762,000	781,558,000
Parent Company | Distribution
Guarantees
Guarantor obligations	405,271,000	383,644,000
Parent Company | Power supply
Guarantees
Guarantor obligations	460,652,000	380,382,000
Parent Company | Statewide and associate
Guarantees
Guarantor obligations	17,839,000	17,532,000
NCSC
Guarantees
Guarantor obligations	16,679,000	15,263,000
Variable interest entity, primary beneficiary
Guarantees
Guarantor obligations	16,679,000	15,263,000
Variable interest entity, primary beneficiary | Statewide and associate
Guarantees
Guarantor obligations	16,000,000	16,000,000
Long-term tax-exempt bonds
Guarantees
Guarantor obligations	97,705,000	98,405,000
Letters of credit
Guarantees
Guarantor obligations	618,172,000	538,393,000
Other guarantees
Guarantees
Guarantor obligations	184,564,000	$ 160,023,000
Master letter of credit | Hybrid letter of credit
Guarantees
Maximum additional amount potentially required to be issued	$ 30,000,000