COMMON STOCK EQUITY - ACCUMULATED OTHER COMPREHENSIVE INCOME (Details) - USD ($) $ in Thousands		3 Months Ended		6 Months Ended
		Mar. 31, 2016	Mar. 31, 2015	Mar. 31, 2016	Mar. 31, 2015
Increase (Decrease) in Accumulated Other Comprehensive Income [Roll Forward]
Balance as of beginning of period				$ 1,106,956
Other comprehensive income (loss)		$ 4,795	$ (193)	8,719	$ 682
Balance as of end of period		1,207,482		1,207,482
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				8,201	109
Amounts reclassified from accumulated other comprehensive income, net of tax				0	0
Other comprehensive income (loss)				8,201	109
Balance as of end of period		14,586	4,891	14,586	4,891
Tax on other comprehensive income (loss) before reclassifications				5,768	75
Tax on amounts reclassified from accumulated other comprehensive income				0	0
Tax on net current-period other comprehensive income (loss)				5,768	75
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				(22)	(256)
Amounts reclassified from accumulated other comprehensive income, net of tax	[1]			27	343
Other comprehensive income (loss)				5	87
Balance as of end of period		5	(6)	5	(6)
Tax on other comprehensive income (loss) before reclassifications				(13)	(149)
Tax on amounts reclassified from accumulated other comprehensive income				15	199
Tax on net current-period other comprehensive income (loss)				2	50
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]			513	486
Other comprehensive income (loss)				513	486
Balance as of end of period		(15,266)	(9,797)	(15,266)	(9,797)
Tax on other comprehensive income (loss) before reclassifications				0	0
Tax on amounts reclassified from accumulated other comprehensive income				349	337
Tax on net current-period other comprehensive income (loss)				349	337
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				8,179	(147)
Amounts reclassified from accumulated other comprehensive income, net of tax				540	829
Other comprehensive income (loss)				8,719	682
Balance as of end of period		$ (675)	$ (4,912)	(675)	(4,912)
Tax on other comprehensive income (loss) before reclassifications				5,755	(74)
Tax on amounts reclassified from accumulated other comprehensive income				364	536
Tax on net current-period other comprehensive income (loss)				$ 6,119	$ 462

[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.