COMMON STOCK EQUITY - ACCUMULATED OTHER COMPREHENSIVE INCOME (Details) - USD ($) $ in Thousands		3 Months Ended
		Dec. 31, 2015	Dec. 31, 2014
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		3,660	644
Amounts reclassified from accumulated other comprehensive income, net of tax		264	231
Other comprehensive income		3,924	875
Balance as of end of period		(5,470)	(4,719)
Tax on other comprehensive income (loss) before reclassifications		2,591	446
Tax on amounts reclassified from accumulated other comprehensive income		178	160
Tax on net current-period other comprehensive income (loss)		2,769	606
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		3,701	659
Amounts reclassified from accumulated other comprehensive income, net of tax		0	0
Other comprehensive income		3,701	659
Balance as of end of period		10,086	5,441
Tax on other comprehensive income (loss) before reclassifications		2,614	455
Tax on amounts reclassified from accumulated other comprehensive income		0	0
Tax on net current-period other comprehensive income (loss)		2,614	455
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		(41)	(15)
Amounts reclassified from accumulated other comprehensive income, net of tax	[1]	8	(16)
Other comprehensive income		(33)	(31)
Balance as of end of period		(33)	(124)
Tax on other comprehensive income (loss) before reclassifications		(23)	(9)
Tax on amounts reclassified from accumulated other comprehensive income		4	(9)
Tax on net current-period other comprehensive income (loss)		(19)	(18)
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]	256	247
Other comprehensive income		256	247
Balance as of end of period		(15,523)	(10,036)
Tax on other comprehensive income (loss) before reclassifications		0	0
Tax on amounts reclassified from accumulated other comprehensive income		174	169
Tax on net current-period other comprehensive income (loss)		$ 174	$ 169

[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.