Subsequent Events (Details) - USD ($) $ in Thousands	Feb. 28, 2022	May 31, 2021	Feb. 28, 2021
Subsequent Event [Line Items]
Total loans outstanding	$ 29,508,363	$ 28,415,107	$ 28,315,671
Parent Company
Subsequent Event [Line Items]
Total loans outstanding	28,320,476	27,287,856
Parent Company | Power supply
Subsequent Event [Line Items]
Total loans outstanding	4,992,073	$ 5,154,312
Parent Company | Brazos Sandy Creek | Power supply
Subsequent Event [Line Items]
Total loans outstanding	$ 28,000