Guarantees - Guarantees Outstanding (Details) - USD ($)	Feb. 28, 2023	May 31, 2022
Guarantees
Guarantor obligations	$ 775,807,000	$ 730,783,000
Parent Company
Guarantees
Guarantor obligations	747,843,000	706,813,000
Parent Company | Distribution
Guarantees
Guarantor obligations	350,864,000	314,925,000
Parent Company | Power supply
Guarantees
Guarantor obligations	379,834,000	378,516,000
Parent Company | Statewide and associate
Guarantees
Guarantor obligations	17,145,000	13,372,000
NCSC
Guarantees
Guarantor obligations	27,964,000	23,970,000
Variable interest entity, primary beneficiary
Guarantees
Guarantor obligations	27,964,000	23,970,000
Variable interest entity, primary beneficiary | Statewide and associate
Guarantees
Guarantor obligations	16,000,000	11,000,000
Long-term tax-exempt bonds
Guarantees
Guarantor obligations	99,600,000	122,150,000
Letters of credit
Guarantees
Guarantor obligations	516,042,000	450,354,000
Other guarantees
Guarantees
Guarantor obligations	160,165,000	$ 158,279,000
Master letter of credit | Hybrid letter of credit
Guarantees
Maximum additional amount potentially required to be issued	$ 31,000,000