SHAREHOLDERS' EQUITY - AOCI Changes (Details) $ in Thousands	12 Months Ended
	Sep. 30, 2018 USD ($)
Rollforward of accumulated other comprehensive income (loss), net of tax
Balance at beginning of period	$ 4,164,591
Balance at end of period	4,382,735
Unrealized Appreciation on Available for sale Securities
Rollforward of accumulated other comprehensive income (loss), net of tax
Balance at beginning of period	20,070
Other comprehensive income before reclassifications	9,001
Net current-period other comprehensive income	9,001
Balance at end of period	29,071
Defined Benefit Pension Plan
Rollforward of accumulated other comprehensive income (loss), net of tax
Balance at beginning of period	(17,770)
Amounts reclassified from accumulated other comprehensive income	5,249
Net current-period other comprehensive income	5,249
Balance at end of period	(12,521)
Accumulated Other Comprehensive (Loss) Income
Rollforward of accumulated other comprehensive income (loss), net of tax
Balance at beginning of period	2,300
Other comprehensive income before reclassifications	9,001
Amounts reclassified from accumulated other comprehensive income	5,249
Net current-period other comprehensive income	14,250
Balance at end of period	$ 16,550