SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES, ACCUMULATED OTHER COMPREHENSIVE INCOME (Details) - USD ($) $ in Thousands		12 Months Ended
		Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2014
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Beginning Balance		$ 1,106,956	$ 966,166	$ 887,384
Other comprehensive (loss)		(5,761)	(3,800)	(3,973)
Ending Balance		1,166,591	1,106,956	966,166
Unrealized gain (loss) on available for sale securities
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Beginning Balance		6,385	4,782
Other comprehensive income (loss), before reclassifications, net of tax		(2,187)	1,603
Amounts reclassified from accumulated other comprehensive income, net of tax		0	0
Other comprehensive (loss)		(2,187)	1,603
Ending Balance		4,198	6,385	4,782
Tax on other comprehensive income before reclassifications		(1,499)	1,135
Tax on amounts reclassified from accumulated other comprehensive income		0	0
Tax on net current-period other comprehensive income (loss)		(1,499)	1,135
Net unrealized gain (loss) on derivatives
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Beginning Balance		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)		0	93
Ending Balance		0	0	(93)
Tax on other comprehensive income 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
Adjustment to postemployment benefit obligation
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Beginning Balance		(15,779)	(10,283)
Other comprehensive income (loss), before reclassifications, net of tax		(4,600)	(6,483)
Amounts reclassified from accumulated other comprehensive income, net of tax	[2]	1,026	987
Other comprehensive (loss)		(3,574)	(5,496)
Ending Balance		(19,353)	(15,779)	(10,283)
Tax on other comprehensive income before reclassifications		(3,164)	(4,362)
Tax on amounts reclassified from accumulated other comprehensive income		698	674
Tax on net current-period other comprehensive income (loss)		(2,466)	(3,688)
Total
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Beginning Balance		(9,394)	(5,594)	(1,621)
Other comprehensive income (loss), before reclassifications, net of tax		(6,804)	(5,136)
Amounts reclassified from accumulated other comprehensive income, net of tax		1,043	1,336
Other comprehensive (loss)		(5,761)	(3,800)	(3,973)
Ending Balance		(15,155)	(9,394)	$ (5,594)
Tax on other comprehensive income before reclassifications		(4,673)	(3,373)
Tax on amounts reclassified from accumulated other comprehensive income		708	876
Tax on net current-period other comprehensive income (loss)		$ (3,965)	$ (2,497)

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

[2] Included in the computation of net periodic pension cost, a component of O&M expense on the Consolidated Statements of Operations. For more details, see Note 10. Employee Benefit Plans.