STOCK-BASED COMPENSATION - STOCK-BASED COMPENSATION EXPENSE (Details) - USD ($) $ in Thousands	12 Months Ended
Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2017
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Compensation expense included in operation and maintenance expense	$ 9,796	$ 12,845	$ 5,807
Income tax benefit	(2,848)	(3,734)	(2,372)
Total, net of tax	6,948	9,111	3,435
Tax benefit of delivered shares from stock based compensation	1,290	2,950	1,285
Performance share awards
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Compensation expense included in operation and maintenance expense	5,804	3,526	2,614
Restricted and non-restricted stock
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Compensation expense included in operation and maintenance expense	2,492	2,191	1,732
Deferred retention stock
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Compensation expense included in operation and maintenance expense	$ 1,500	$ 7,128	$ 1,461

Amount of expense for award under share-based payment arrangement. Excludes amount capitalized.

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