COMMON STOCK EQUITY - ACCUMULATED OTHER COMPREHENSIVE INCOME (LOSS) (Details) - USD ($) $ in Thousands	3 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Increase (Decrease) in Accumulated Other Comprehensive Income [Roll Forward]
Balance as of beginning of period	$ 1,236,643
Other comprehensive income (loss), before reclassifications, net of tax	(2,290)	$ 7,042
Amounts reclassified from accumulated other comprehensive income, net of tax	(2,914)	(1,210)
Other comprehensive (loss) income	(5,204)	5,832
Balance as of end of period	1,347,770
Available for Sale Securities
Increase (Decrease) in Accumulated Other Comprehensive Income [Roll Forward]
Balance as of beginning of period	11,044	4,198
Other comprehensive income (loss), before reclassifications, net of tax	(2,290)	7,042
Amounts reclassified from accumulated other comprehensive income, net of tax	(3,154)	(1,527)
Other comprehensive (loss) income	(5,444)	5,515
Balance as of end of period	5,600	9,713
Tax on other comprehensive income (loss) before reclassifications	(851)	4,840
Tax on amounts reclassified from accumulated other comprehensive income	(2,178)	(1,054)
Tax on net current-period other comprehensive income (loss)	(3,029)	3,786
Postemployment Benefit Obligation
Increase (Decrease) in Accumulated Other Comprehensive Income [Roll Forward]
Balance as of beginning of period	(14,300)	(19,353)
Other comprehensive income (loss), before reclassifications, net of tax	0	0
Amounts reclassified from accumulated other comprehensive income, net of tax	240	317
Other comprehensive (loss) income	240	317
Balance as of end of period	(14,060)	(19,036)
Tax on other comprehensive income (loss) before reclassifications	0	0
Tax on amounts reclassified from accumulated other comprehensive income	136	217
Tax on net current-period other comprehensive income (loss)	136	217
Total
Increase (Decrease) in Accumulated Other Comprehensive Income [Roll Forward]
Balance as of beginning of period	(3,256)	(15,155)
Balance as of end of period	(8,460)	(9,323)
Tax on other comprehensive income (loss) before reclassifications	(851)	4,840
Tax on amounts reclassified from accumulated other comprehensive income	(2,042)	(837)
Tax on net current-period other comprehensive income (loss)	$ (2,893)	$ 4,003