COMMON STOCK EQUITY - ACCUMULATED OTHER COMPREHENSIVE INCOME (LOSS) (Details) - USD ($) $ in Thousands	3 Months Ended
	Dec. 31, 2016	Dec. 31, 2015
Increase (Decrease) in Accumulated Other Comprehensive Income [Roll Forward]
Balance as of beginning of period	$ 1,166,591
Other comprehensive income	5,832	$ 3,924
Balance as of end of period	1,185,361
Available for Sale Securities
Increase (Decrease) in Accumulated Other Comprehensive Income [Roll Forward]
Balance as of beginning of period	4,198	6,385
Other comprehensive income (loss), before reclassifications, net of tax	7,042	3,701
Amounts reclassified from accumulated other comprehensive income, net of tax	(1,527)	0
Other comprehensive income	5,515	3,701
Balance as of end of period	9,713	10,086
Tax on other comprehensive income (loss) before reclassifications	4,840	2,614
Tax on amounts reclassified from accumulated other comprehensive income	(1,054)	0
Tax on net current-period other comprehensive income (loss)	3,786	2,614
Cash Flow Hedges
Increase (Decrease) in Accumulated Other Comprehensive Income [Roll Forward]
Balance as of beginning of period	0	0
Other comprehensive income (loss), before reclassifications, net of tax	0	(41)
Amounts reclassified from accumulated other comprehensive income, net of tax	0	8
Other comprehensive income	0	(33)
Balance as of end of period	0	(33)
Tax on other comprehensive income (loss) before reclassifications	0	(23)
Tax on amounts reclassified from accumulated other comprehensive income	0	4
Tax on net current-period other comprehensive income (loss)	0	(19)
Postemployment Benefit Obligation
Increase (Decrease) in Accumulated Other Comprehensive Income [Roll Forward]
Balance as of beginning of period	(19,353)	(15,779)
Other comprehensive income (loss), before reclassifications, net of tax	0	0
Amounts reclassified from accumulated other comprehensive income, net of tax	317	256
Other comprehensive income	317	256
Balance as of end of period	(19,036)	(15,523)
Tax on other comprehensive income (loss) before reclassifications	0	0
Tax on amounts reclassified from accumulated other comprehensive income	217	174
Tax on net current-period other comprehensive income (loss)	217	174
Total
Increase (Decrease) in Accumulated Other Comprehensive Income [Roll Forward]
Balance as of beginning of period	(15,155)	(9,394)
Other comprehensive income (loss), before reclassifications, net of tax	7,042	3,660
Amounts reclassified from accumulated other comprehensive income, net of tax	(1,210)	264
Other comprehensive income	5,832	3,924
Balance as of end of period	(9,323)	(5,470)
Tax on other comprehensive income (loss) before reclassifications	4,840	2,591
Tax on amounts reclassified from accumulated other comprehensive income	(837)	178
Tax on net current-period other comprehensive income (loss)	$ 4,003	$ 2,769