SHAREHOLDERS' EQUITY - Components of Accumulated Other Comprehensive Income (Loss) (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2017
After-tax amounts:
Accumulated other comprehensive income (loss), after-tax	$ (28,635)	$ 16,550
Unrealized Appreciation on Equity Securities
Pre-tax amounts:
Accumulated other comprehensive income (loss), pre-tax	0	44,023	$ 31,700
After-tax amounts:
Accumulated other comprehensive income (loss), after-tax	0	29,071	20,070
Unrealized Actuarial Loss
Pre-tax amounts:
Accumulated other comprehensive income (loss), pre-tax	(37,084)	(21,693)	(28,873)
After-tax amounts:
Accumulated other comprehensive income (loss), after-tax	(28,635)	(12,521)	(17,770)
Accumulated Other Comprehensive Income (Loss)
Pre-tax amounts:
Accumulated other comprehensive income (loss), pre-tax	(37,084)	22,330	2,827
After-tax amounts:
Accumulated other comprehensive income (loss), after-tax	$ (28,635)	$ 16,550	$ 2,300