SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - ACCUMULATED OTHER COMPREHENSIVE (LOSS) INCOME (Details) - USD ($) $ in Thousands	3 Months Ended
	Dec. 31, 2021	Dec. 31, 2020
Increase (Decrease) in Accumulated Other Comprehensive Income [Roll Forward]
Balance as of beginning of period	$ 1,630,862	$ 1,643,896
Balance as of end of period	1,716,978	1,698,214
Cash Flow Hedges
Increase (Decrease) in Accumulated Other Comprehensive Income [Roll Forward]
Balance as of beginning of period	(9,376)	(10,397)
Amounts reclassified from accumulated other comprehensive income, net of tax	264	231
Balance as of end of period	(9,112)	(10,166)
Tax on amounts reclassified from accumulated other comprehensive income	(79)	(112)
Postemployment Benefit Obligation
Increase (Decrease) in Accumulated Other Comprehensive Income [Roll Forward]
Balance as of beginning of period	(25,152)	(33,918)
Amounts reclassified from accumulated other comprehensive income, net of tax	766	812
Balance as of end of period	(24,386)	(33,106)
Tax on amounts reclassified from accumulated other comprehensive income	(233)	(245)
Total
Increase (Decrease) in Accumulated Other Comprehensive Income [Roll Forward]
Balance as of beginning of period	(34,528)	(44,315)
Amounts reclassified from accumulated other comprehensive income, net of tax	1,030	1,043
Balance as of end of period	(33,498)	(43,272)
Tax on amounts reclassified from accumulated other comprehensive income	$ (312)	$ (357)