SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - IMPACT OF ASC 740 ON THE UNAUDITED CONDENSED CONSOLIDATED STATEMENTS OF COMMON STOCK EQUITY (Details) - USD ($) $ in Thousands	3 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Balance as of beginning of period	$ 1,643,896	$ 1,345,615
Net income	81,045	75,752
Balance as of end of period	1,698,214	1,608,092
Retained Earnings
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Balance as of beginning of period	947,501	869,858
Net income	81,045	75,752
Balance as of end of period	996,580	915,764
As Previously Reported
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Balance as of beginning of period	1,844,692	1,551,717
Net income		89,361
Balance as of end of period		1,827,803
As Previously Reported | Retained Earnings
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Balance as of beginning of period		1,075,960
Net income		89,361
Balance as of end of period		1,135,475
Effect of Change
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Balance as of beginning of period	$ (200,796)	(206,102)
Net income		(13,609)
Balance as of end of period		(219,711)
Effect of Change | Retained Earnings
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Balance as of beginning of period		(206,102)
Net income		(13,609)
Balance as of end of period		$ (219,711)