SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - IMPACT OF ASC 740 ON THE UNAUDITED CONDENSED CONSOLIDATED STATEMENTS OF COMMON STOCK EQUITY (Details) - USD ($) $ in Thousands	12 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2019
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Beginning Balance	$ 1,643,896	$ 1,381,833	$ 1,294,664
Net income	117,890	163,007	123,935
Ending Balance	1,630,862	1,643,896	1,381,833
As Previously Reported
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Beginning Balance	1,844,692	1,551,717	1,418,978
Net income		193,919	169,505
Ending Balance		1,844,692	1,551,717
Effect of Change
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Beginning Balance	(200,796)	(169,884)	(124,314)
Net income		(30,912)	(45,570)
Ending Balance		(200,796)	(169,884)
Retained Earnings
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Beginning Balance	947,501	906,076	882,803
Net income	117,890	163,007	123,935
Ending Balance	934,610	947,501	906,076
Retained Earnings | As Previously Reported
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Beginning Balance	1,148,297	1,075,960	1,007,117
Net income		193,919	169,505
Ending Balance		1,148,297	1,075,960
Retained Earnings | Effect of Change
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Beginning Balance	$ (200,796)	(169,884)	(124,314)
Net income		(30,912)	(45,570)
Ending Balance		$ (200,796)	$ (169,884)