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				6 Months Ended
	Mar. 31, 2021	Dec. 31, 2020	Mar. 31, 2020	Dec. 31, 2019	Mar. 31, 2021	Mar. 31, 2020
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Balance as of beginning of period	$ 1,698,214	$ 1,643,896	$ 1,608,092	$ 1,345,615	$ 1,643,896	$ 1,345,615
Net income	149,809	81,045	73,846	75,752	230,854	149,598
Balance as of end of period	1,821,833	1,698,214	1,654,737	1,608,092	1,821,833	1,654,737
Retained Earnings
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Balance as of beginning of period	996,580	947,501	915,764	869,858	947,501	869,858
Net income	149,809	81,045	73,846	75,752
Balance as of end of period	$ 1,114,382	996,580	959,722	915,764	1,114,382	959,722
As Previously Reported
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Balance as of beginning of period		1,844,692	1,827,803	1,551,717	1,844,692	1,551,717
Net income			88,505	89,361		177,866
Balance as of end of period			1,889,107	1,827,803		1,889,107
As Previously Reported | Retained Earnings
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Balance as of beginning of period			1,135,475	1,075,960		1,075,960
Net income			88,505	89,361
Balance as of end of period			1,194,092	1,135,475		1,194,092
Effect of Change
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Balance as of beginning of period		$ (200,796)	(219,711)	(206,102)	$ (200,796)	(206,102)
Net income			(14,659)	(13,609)		(28,268)
Balance as of end of period			(234,370)	(219,711)		(234,370)
Effect of Change | Retained Earnings
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Balance as of beginning of period			(219,711)	(206,102)		(206,102)
Net income			(14,659)	(13,609)
Balance as of end of period			$ (234,370)	$ (219,711)		$ (234,370)