SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - ACCUMULATED OTHER COMPREHENSIVE LOSS (Details) - USD ($) $ in Thousands	3 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Oct. 01, 2018
Increase (Decrease) in Accumulated Other Comprehensive Income [Roll Forward]
Balance as of beginning of period	$ 1,551,717	$ 1,418,978
Balance as of end of period	1,827,803	1,497,050
Investments in Equity Securities
Increase (Decrease) in Accumulated Other Comprehensive Income [Roll Forward]
Balance as of beginning of period	0	3,446
Amounts reclassified from accumulated other comprehensive income, net of tax	0	0
Reclassification to retained earnings			$ (3,446)
Balance as of end of period	0	0
Tax on amounts reclassified from accumulated other comprehensive income	0	0
Postemployment Benefit Obligation
Increase (Decrease) in Accumulated Other Comprehensive Income [Roll Forward]
Balance as of beginning of period	(31,787)	(16,056)
Amounts reclassified from accumulated other comprehensive income, net of tax	759	234
Reclassification to retained earnings			0
Balance as of end of period	(31,028)	(15,822)
Tax on amounts reclassified from accumulated other comprehensive income	(217)	(96)
Total
Increase (Decrease) in Accumulated Other Comprehensive Income [Roll Forward]
Balance as of beginning of period	(31,787)	(12,610)
Amounts reclassified from accumulated other comprehensive income, net of tax	759	234
Reclassification to retained earnings			$ (3,446)
Balance as of end of period	(31,028)	(15,822)
Tax on amounts reclassified from accumulated other comprehensive income	$ (217)	$ (96)