Guarantees - Guarantees Outstanding (Details) - USD ($) $ in Thousands	Feb. 28, 2022	May 31, 2021
Guarantees
Guarantor obligations	$ 707,304	$ 689,080
Parent Company
Guarantees
Guarantor obligations	681,862	672,530
Parent Company | Distribution
Guarantees
Guarantor obligations	297,577	251,023
Parent Company | Power supply
Guarantees
Guarantor obligations	372,048	415,984
Parent Company | Statewide and associate
Guarantees
Guarantor obligations	12,237	5,523
NCSC
Guarantees
Guarantor obligations	25,442	16,550
Variable interest entity, primary beneficiary
Guarantees
Guarantor obligations	25,442	16,550
Variable interest entity, primary beneficiary | Statewide and associate
Guarantees
Guarantor obligations	10,000	3,000
Long-term tax-exempt bonds
Guarantees
Guarantor obligations	123,775	145,025
Letters of credit
Guarantees
Guarantor obligations	425,917	389,735
Other guarantees
Guarantees
Guarantor obligations	$ 157,612	$ 154,320