SHAREHOLDERS’ EQUITY - AOCI Changes (Details) $ in Thousands	12 Months Ended
	Sep. 30, 2017 USD ($)
Rollforward of accumulated other comprehensive income (loss), net of tax
Balance at beginning of period	$ 4,560,925
Balance at end of period	4,164,591
Unrealized appreciation (depreciation) on securities
Rollforward of accumulated other comprehensive income (loss), net of tax
Balance at beginning of period	20,899
Other comprehensive loss before reclassifications	(829)
Net current-period other comprehensive income (loss)	(829)
Balance at end of period	20,070
Defined benefit pension plan
Rollforward of accumulated other comprehensive income (loss), net of tax
Balance at beginning of period	(21,103)
Amounts reclassified from accumulated other comprehensive income	3,333
Net current-period other comprehensive income (loss)	3,333
Balance at end of period	(17,770)
Accumulated Other Comprehensive Loss
Rollforward of accumulated other comprehensive income (loss), net of tax
Balance at beginning of period	(204)
Other comprehensive loss before reclassifications	(829)
Amounts reclassified from accumulated other comprehensive income	3,333
Net current-period other comprehensive income (loss)	2,504
Balance at end of period	$ 2,300