STOCK-BASED COMPENSATION - STOCK-BASED COMPENSATION EXPENSE (Details) - USD ($) $ in Thousands	12 Months Ended
	Sep. 30, 2025	Sep. 30, 2024	Sep. 30, 2023
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Compensation expense included in operation and maintenance expense	$ 17,779	$ 16,028	$ 14,716
Income tax benefit	(4,310)	(3,898)	(3,563)
Total, net of tax	13,469	12,130	11,153
Taxes related to stock-based compensation	1,934	1,219	588
Performance share awards
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Compensation expense included in operation and maintenance expense	6,679	5,437	4,882
Restricted and non-restricted stock
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Compensation expense included in operation and maintenance expense	4,174	3,958	3,647
Deferred retention stock
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Compensation expense included in operation and maintenance expense	$ 6,926	$ 6,633	$ 6,187