SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - ACCUMULATED OTHER COMPREHENSIVE (LOSS) INCOME (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Mar. 31, 2022	Mar. 31, 2021	Mar. 31, 2022	Mar. 31, 2021
Increase (Decrease) in Accumulated Other Comprehensive Income [Roll Forward]
Balance as of beginning of period	$ 1,716,978	$ 1,698,214	$ 1,630,862	$ 1,643,896
Balance as of end of period	1,784,287	1,821,833	1,784,287	1,821,833
Cash Flow Hedges
Increase (Decrease) in Accumulated Other Comprehensive Income [Roll Forward]
Balance as of beginning of period	(9,112)	(10,166)	(9,376)	(10,397)
Amounts reclassified from accumulated other comprehensive income, net of tax	263	263	527	494
Balance as of end of period	(8,849)	(9,903)	(8,849)	(9,903)
Tax on amounts reclassified from accumulated other comprehensive income	(80)	(79)	(159)	(191)
Postemployment Benefit Obligation
Increase (Decrease) in Accumulated Other Comprehensive Income [Roll Forward]
Balance as of beginning of period	(24,386)	(33,106)	(25,152)	(33,918)
Amounts reclassified from accumulated other comprehensive income, net of tax	768	813	1,534	1,625
Balance as of end of period	(23,618)	(32,293)	(23,618)	(32,293)
Tax on amounts reclassified from accumulated other comprehensive income	(232)	(244)	(465)	(489)
Total
Increase (Decrease) in Accumulated Other Comprehensive Income [Roll Forward]
Balance as of beginning of period	(33,498)	(43,272)	(34,528)	(44,315)
Amounts reclassified from accumulated other comprehensive income, net of tax	1,031	1,076	2,061	2,119
Balance as of end of period	(32,467)	(42,196)	(32,467)	(42,196)
Tax on amounts reclassified from accumulated other comprehensive income	$ (312)	$ (323)	$ (624)	$ (680)