SHAREHOLDERS' EQUITY - AOCI Components (Details) - USD ($) $ in Thousands	12 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2014
Pre-tax amounts:
Accumulated other comprehensive income (loss) before tax	$ (1,061)	$ (3,123)	$ 134,433
Rollforward of accumulated other comprehensive income (loss), net of tax
Balance	4,895,846	4,891,169	4,446,075
Balance	4,560,925	4,895,846	4,891,169
Accumulated other comprehensive income (loss)
Rollforward of accumulated other comprehensive income (loss), net of tax
Balance	(1,377)	83,126	132,530
Other comprehensive income before reclassifications	2,772
Amounts reclassified from accumulated other comprehensive income (loss)	(1,599)	(4,286)
Net current-period other comprehensive Income (loss)	1,173
Balance	(204)	(1,377)	83,126
Unrealized appreciation (depreciation) on securities
Pre-tax amounts:
Accumulated other comprehensive income (loss) before tax	33,051	27,021	157,838
Rollforward of accumulated other comprehensive income (loss), net of tax
Balance	17,201	97,418
Other comprehensive income before reclassifications	2,772
Amounts reclassified from accumulated other comprehensive income (loss)	926
Net current-period other comprehensive Income (loss)	3,698
Balance	20,899	17,201	97,418
Defined benefit pension plan
Pre-tax amounts:
Accumulated other comprehensive income (loss) before tax	(34,112)	(30,144)	(23,405)
Rollforward of accumulated other comprehensive income (loss), net of tax
Balance	(18,578)	(14,292)
Amounts reclassified from accumulated other comprehensive income (loss)	(2,525)	(4,286)
Net current-period other comprehensive Income (loss)	(2,525)
Balance	$ (21,103)	$ (18,578)	$ (14,292)