BUSINESS SEGMENT AND OTHER OPERATIONS DATA - NET FINANCIAL EARNINGS LOSS RECONCILIATION (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2015
Segment Reporting [Abstract]
Consolidated net financial earnings									$ 149,392	$ 138,085	$ 151,503
Less:
Unrealized (gain) loss on derivative instruments and related transactions									(11,241)	46,883	(38,681)
Unrealized Gain (Loss) on Derivatives and Commodity Contracts, Tax									4,062	(17,018)	14,391
Effects of economic hedging related to natural gas inventory									38,470	(36,816)	(8,225)
Tax effect									(13,964)	13,364	3,058
NET INCOME	$ (36,523)	$ 18,957	$ 114,702	$ 34,929	$ 25,400	$ (17,363)	$ 73,354	$ 50,281	$ 132,065	$ 131,672	$ 180,960