COMMON STOCK EQUITY - ACCUMULATED OTHER COMPREHENSIVE INCOME (LOSS) (Details) - USD ($) $ in Thousands		3 Months Ended		9 Months Ended
		Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015
Increase (Decrease) in Accumulated Other Comprehensive Income [Roll Forward]
Balance as of beginning of period				$ 1,106,956
Other comprehensive (loss) income		$ (5,062)	$ (318)	3,657	$ 364
Balance as of end of period		1,170,882		1,170,882
Available for Sale Securities [Member]
Increase (Decrease) in Accumulated Other Comprehensive Income [Roll Forward]
Balance as of beginning of period				6,385	4,782
Other comprehensive income (loss), before reclassifications, net of tax				2,888	(461)
Amounts reclassified from accumulated other comprehensive income, net of tax				0	0
Other comprehensive (loss) income				2,888	(461)
Balance as of end of period		9,273	4,321	9,273	4,321
Tax on other comprehensive income (loss) before reclassifications				2,014	(319)
Tax on amounts reclassified from accumulated other comprehensive income				0	0
Tax on net current-period other comprehensive income (loss)				2,014	(319)
Cash Flow Hedges [Member]
Increase (Decrease) in Accumulated Other Comprehensive Income [Roll Forward]
Balance as of beginning of period				0	(93)
Other comprehensive income (loss), before reclassifications, net of tax				(17)	(256)
Amounts reclassified from accumulated other comprehensive income, net of tax	[1]			17	349
Other comprehensive (loss) income				0	93
Balance as of end of period		0	0	0	0
Tax on other comprehensive income (loss) before reclassifications				(10)	(146)
Tax on amounts reclassified from accumulated other comprehensive income				10	202
Tax on net current-period other comprehensive income (loss)				0	56
Postemployment Benefit Obligation [Member]
Increase (Decrease) in Accumulated Other Comprehensive Income [Roll Forward]
Balance as of beginning of period				(15,779)	(10,283)
Other comprehensive income (loss), before reclassifications, net of tax				0	0
Amounts reclassified from accumulated other comprehensive income, net of tax	[2]			769	732
Other comprehensive (loss) income				769	732
Balance as of end of period		(15,010)	(9,551)	(15,010)	(9,551)
Tax on other comprehensive income (loss) before reclassifications				0	0
Tax on amounts reclassified from accumulated other comprehensive income				523	506
Tax on net current-period other comprehensive income (loss)				523	506
Total [Member]
Increase (Decrease) in Accumulated Other Comprehensive Income [Roll Forward]
Balance as of beginning of period				(9,394)	(5,594)
Other comprehensive income (loss), before reclassifications, net of tax				2,871	(717)
Amounts reclassified from accumulated other comprehensive income, net of tax				786	1,081
Other comprehensive (loss) income				3,657	364
Balance as of end of period		$ (5,737)	$ (5,230)	(5,737)	(5,230)
Tax on other comprehensive income (loss) before reclassifications				2,004	(465)
Tax on amounts reclassified from accumulated other comprehensive income				533	708
Tax on net current-period other comprehensive income (loss)				$ 2,537	$ 243

[1] Consists of realized losses related to foreign currency derivatives, which are reclassified to gas purchases on the Unaudited Condensed Consolidated Statements of Operations.

[2] Included in the computation of net periodic pension cost, a component of operations and maintenance expense on the Unaudited Condensed Consolidated Statements of Operations.