COMMON STOCK EQUITY - ACCUMULATED OTHER COMPREHENSIVE INCOME (LOSS) (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Mar. 31, 2018	Mar. 31, 2017
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	$ (20,401)	$ (3,054)	(22,691)	$ 10,096
Amounts reclassified from accumulated other comprehensive income, net of tax	15,073	1,328	12,159	(2,538)
Other comprehensive (loss) income	(5,328)	(1,726)	(10,532)	7,558
Balance as of end of period	1,467,374		1,467,374
Available for Sale Securities
Increase (Decrease) in Accumulated Other Comprehensive Income [Roll Forward]
Balance as of beginning of period	5,600	9,713	11,044	4,198
Other comprehensive income (loss), before reclassifications, net of tax	(20,401)	(3,054)	(22,691)	10,096
Amounts reclassified from accumulated other comprehensive income, net of tax	14,801	1,646	11,647	(3,173)
Other comprehensive (loss) income	(5,600)	(1,408)	(11,044)	6,923
Balance as of end of period	0	11,121	0	11,121
Tax on other comprehensive income (loss) before reclassifications	(7,366)	1,808	(8,217)	6,648
Tax on amounts reclassified from accumulated other comprehensive income	3,036	(1,138)	858	(2,192)
Tax on net current-period other comprehensive income (loss)	(4,330)	670	(7,359)	4,456
Postemployment Benefit Obligation
Increase (Decrease) in Accumulated Other Comprehensive Income [Roll Forward]
Balance as of beginning of period	(14,060)	(19,036)	(14,300)	(19,353)
Other comprehensive income (loss), before reclassifications, net of tax	0	0	0	0
Amounts reclassified from accumulated other comprehensive income, net of tax	272	(318)	512	635
Other comprehensive (loss) income	272	(318)	512	635
Balance as of end of period	(13,788)	(18,718)	(13,788)	(18,718)
Tax on other comprehensive income (loss) before reclassifications	0	0	0	0
Tax on amounts reclassified from accumulated other comprehensive income	104	217	240	434
Tax on net current-period other comprehensive income (loss)	104	217	240	434
Total
Increase (Decrease) in Accumulated Other Comprehensive Income [Roll Forward]
Balance as of beginning of period	(8,460)	(9,323)	(3,256)	(15,155)
Balance as of end of period	(13,788)	(7,597)	(13,788)	(7,597)
Tax on other comprehensive income (loss) before reclassifications	(7,366)	1,808	(8,217)	6,648
Tax on amounts reclassified from accumulated other comprehensive income	3,140	(921)	1,098	(1,758)
Tax on net current-period other comprehensive income (loss)	$ (4,226)	$ 887	$ (7,119)	$ 4,890