SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - ACCUMULATED OTHER COMPREHENSIVE INCOME (Details) - USD ($) $ in Thousands	12 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2018
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Beginning Balance	$ 1,643,896	$ 1,381,833
Common stock equity	1,630,862	1,643,896	$ 1,294,664
Ending Balance	1,630,862	1,643,896
Cash Flow Hedges
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Beginning Balance	(10,397)	0
Other comprehensive (loss), before reclassifications, net of tax	0	(10,505)
Amounts reclassified from accumulated other comprehensive income, net of tax	1,021	108
Net current-period other comprehensive (loss), net of tax	1,021	(10,397)
Common stock equity	(9,376)	(10,397)
Ending Balance	(9,376)	(10,397)
Tax on other comprehensive income before reclassifications	0	3,203
Tax on amounts reclassified from accumulated other comprehensive income	(350)	(32)
Tax on net current-period other comprehensive income	(350)	3,171
Postemployment Benefit Obligation
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Beginning Balance	(33,918)	(31,787)
Other comprehensive (loss), before reclassifications, net of tax	5,494	(4,882)
Amounts reclassified from accumulated other comprehensive income, net of tax	3,272	2,751
Net current-period other comprehensive (loss), net of tax	8,766	(2,131)
Common stock equity	(25,152)	(33,918)
Ending Balance	(25,152)	(33,918)
Tax on other comprehensive income before reclassifications	(1,618)	1,235
Tax on amounts reclassified from accumulated other comprehensive income	(957)	(668)
Tax on net current-period other comprehensive income	(2,575)	567
Total
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Beginning Balance	(44,315)	(31,787)
Other comprehensive (loss), before reclassifications, net of tax	5,494	(15,387)
Amounts reclassified from accumulated other comprehensive income, net of tax	4,293	2,859
Net current-period other comprehensive (loss), net of tax	9,787	(12,528)
Common stock equity	(34,528)	(44,315)	$ (12,610)
Ending Balance	(34,528)	(44,315)
Tax on other comprehensive income before reclassifications	(1,618)	4,438
Tax on amounts reclassified from accumulated other comprehensive income	(1,307)	(700)
Tax on net current-period other comprehensive income	$ (2,925)	$ 3,738