SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - ACCUMULATED OTHER COMPREHENSIVE INCOME (Details) - USD ($) $ in Thousands	12 Months Ended
	Sep. 30, 2017	Sep. 30, 2016
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Beginning Balance	$ 1,166,591	$ 1,106,956
Other comprehensive (loss), before reclassifications, net of tax	13,802	(6,804)
Amounts reclassified from accumulated other comprehensive income, net of tax	(1,903)	1,043
Net current-period other comprehensive (loss), net of tax	11,899	(5,761)
Ending Balance	1,236,643	1,166,591
Unrealized gain (loss) on available for sale securities
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Beginning Balance	4,198	6,385
Other comprehensive (loss), before reclassifications, net of tax	10,019	(2,187)
Amounts reclassified from accumulated other comprehensive income, net of tax	(3,173)	0
Net current-period other comprehensive (loss), net of tax	6,846	(2,187)
Ending Balance	11,044	4,198
Tax on other comprehensive income before reclassifications	6,593	(1,499)
Tax on amounts reclassified from accumulated other comprehensive income	(2,192)	0
Tax on net current-period other comprehensive income (loss)	4,401	(1,499)
Net unrealized gain (loss) on derivatives
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Beginning Balance	0	0
Other comprehensive (loss), before reclassifications, net of tax	0	(17)
Amounts reclassified from accumulated other comprehensive income, net of tax	0	17
Net current-period other comprehensive (loss), net of tax	0	0
Ending Balance	0	0
Tax on other comprehensive income before reclassifications	0	(10)
Tax on amounts reclassified from accumulated other comprehensive income	0	10
Tax on net current-period other comprehensive income (loss)	0	0
Adjustment to postemployment benefit obligation
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Beginning Balance	(19,353)	(15,779)
Other comprehensive (loss), before reclassifications, net of tax	3,783	(4,600)
Amounts reclassified from accumulated other comprehensive income, net of tax	1,270	1,026
Net current-period other comprehensive (loss), net of tax	5,053	(3,574)
Ending Balance	(14,300)	(19,353)
Tax on other comprehensive income before reclassifications	2,619	(3,164)
Tax on amounts reclassified from accumulated other comprehensive income	868	698
Tax on net current-period other comprehensive income (loss)	3,487	(2,466)
Total
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Beginning Balance	(15,155)	(9,394)
Ending Balance	(3,256)	(15,155)
Tax on other comprehensive income before reclassifications	9,212	(4,673)
Tax on amounts reclassified from accumulated other comprehensive income	(1,324)	708
Tax on net current-period other comprehensive income (loss)	$ 7,888	$ (3,965)