INCOME TAXES - DEFERRED TAX ASSETS AND LIABILITIES (Details) - USD ($) $ in Thousands	Sep. 30, 2017	Sep. 30, 2016
Deferred tax assets
Investment tax credits	$ 111,642	$ 76,517
Deferred service contract revenue	3,877	3,601
Incentive compensation	6,260	8,128
Fair value of derivatives	11,519	1,179
Federal net operating losses	28,487	27,541
State net operating losses	23,597	18,113
Overrecovered gas costs	0	3,831
Other	13,845	11,668
Total deferred tax assets	199,227	150,578
Deferred tax liabilities
Property related items	(620,850)	(532,027)
Remediation costs	(11,625)	(7,928)
Equity investments	(38,370)	(37,740)
Postemployment benefits	(6,855)	(7,902)
Conservation incentive plan	(7,195)	(14,953)
Underrecovered gas costs	(4,035)	0
Other	(16,643)	(14,610)
Total deferred tax liabilities	(705,573)	(615,160)
Total net deferred tax liabilities	(506,346)	(464,582)
Natural Gas Distribution
Deferred tax liabilities
Tax credit carryforward	2,300	2,500
Clean Energy Ventures
Deferred tax liabilities
Tax credit carryforward	$ 109,300	$ 74,000