SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - ACCUMULATED OTHER COMPREHENSIVE LOSS (Details) - USD ($) $ in Thousands	3 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Increase (Decrease) in Accumulated Other Comprehensive Income [Roll Forward]
Balance as of beginning of period	$ 1,643,896	$ 1,345,615
Reclassification from AOCI, Current Period, Net of Tax, Attributable to Parent	1,043	759
Balance as of end of period	1,698,214	1,608,092
Cash Flow Hedges
Increase (Decrease) in Accumulated Other Comprehensive Income [Roll Forward]
Balance as of beginning of period	(10,397)	0
Reclassification from AOCI, Current Period, Net of Tax, Attributable to Parent	231	0
Balance as of end of period	(10,166)	0
Tax on amounts reclassified from accumulated other comprehensive income	(112)	0
Postemployment Benefit Obligation
Increase (Decrease) in Accumulated Other Comprehensive Income [Roll Forward]
Balance as of beginning of period	(33,918)	(31,787)
Reclassification from AOCI, Current Period, Net of Tax, Attributable to Parent	812	759
Balance as of end of period	(33,106)	(31,028)
Tax on amounts reclassified from accumulated other comprehensive income	(245)	(217)
Total
Increase (Decrease) in Accumulated Other Comprehensive Income [Roll Forward]
Balance as of beginning of period	(44,315)	(31,787)
Balance as of end of period	(43,272)	(31,028)
Tax on amounts reclassified from accumulated other comprehensive income	$ (357)	$ (217)