SHAREHOLDERS' EQUITY - AOCI Changes (Details) - USD ($) $ in Thousands		3 Months Ended	6 Months Ended
	Oct. 01, 2018	Mar. 31, 2019	Mar. 31, 2019
Rollforward of accumulated other comprehensive income (loss), net of tax
Balance at beginning of period	$ 4,382,735	$ 4,326,238	$ 4,382,735
Balance at end of period		4,318,620	4,318,620
ASU 2016-01 | Adjustments
Rollforward of accumulated other comprehensive income (loss), net of tax
Adoption of ASU No. 2016-01 (1)	(29,100)
Unrealized Appreciation on Equity Securities | As Reported
Rollforward of accumulated other comprehensive income (loss), net of tax
Balance at beginning of period	29,071		29,071
Unrealized Appreciation on Equity Securities | ASU 2016-01 | Adjustments
Rollforward of accumulated other comprehensive income (loss), net of tax
Adoption of ASU No. 2016-01 (1)	(29,071)
Defined Benefit Pension Plan
Rollforward of accumulated other comprehensive income (loss), net of tax
Amounts reclassified from accumulated other comprehensive income		224	449
Net current-period other comprehensive income		224	449
Balance at end of period	(12,521)	(12,072)	(12,072)
Defined Benefit Pension Plan | As Reported
Rollforward of accumulated other comprehensive income (loss), net of tax
Balance at beginning of period	(12,521)	(12,296)	(12,521)
Accumulated Other Comprehensive Income
Rollforward of accumulated other comprehensive income (loss), net of tax
Balance at beginning of period	16,550	(12,296)	16,550
Amounts reclassified from accumulated other comprehensive income			449
Net current-period other comprehensive income			449
Balance at end of period	(12,521)	$ (12,072)	(12,072)
Accumulated Other Comprehensive Income | As Reported
Rollforward of accumulated other comprehensive income (loss), net of tax
Balance at beginning of period	16,550		$ 16,550
Accumulated Other Comprehensive Income | ASU 2016-01 | Adjustments
Rollforward of accumulated other comprehensive income (loss), net of tax
Adoption of ASU No. 2016-01 (1)	$ (29,071)