INFORMATION BY OPERATING SEGMENT (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended		6 Months Ended
	Mar. 31, 2014	Mar. 31, 2013	Mar. 31, 2014	Mar. 31, 2013	Sep. 30, 2013
Operating Segment Information [Abstract]
Operating revenues	$ 638,745	$ 364,145	$ 1,073,997	$ 616,079
Net economic earnings	44,924	30,404	80,550	56,205
Total assets	3,036,185	1,822,119	3,036,185	1,822,119	2,981,016
Reconciliation of Net Economic Earnings to Net Income [Abstract]
Total Net Economic Earnings above	44,924	30,404	80,550	56,205
Add: Unrealized gain (loss) on energy-related derivative contracts, net of tax	51	32	(3)	91
Add: Acquisition, divestiture, and restructuring activities, net of tax	641	597	1,028	597
Net Income	44,232	29,775	79,525	55,517
Gas Utility [Member]
Operating Segment Information [Abstract]
Operating revenues	638,718	363,911	1,073,946	614,702
Net economic earnings	44,727	30,197	80,504	55,538
Total assets	3,036,185	1,821,372	3,036,185	1,821,372
Reconciliation of Net Economic Earnings to Net Income [Abstract]
Total Net Economic Earnings above	44,727	30,197	80,504	55,538
Other [Member]
Operating Segment Information [Abstract]
Operating revenues	27	234	51	1,377
Net economic earnings	197	207	46	667
Total assets	0	747	0	747
Reconciliation of Net Economic Earnings to Net Income [Abstract]
Total Net Economic Earnings above	197	207	46	667
Eliminations [Member]
Operating Segment Information [Abstract]
Operating revenues	0	0	0	0
Net economic earnings	0	0	0	0
Total assets	0	0	0	0
Reconciliation of Net Economic Earnings to Net Income [Abstract]
Total Net Economic Earnings above	$ 0	$ 0	$ 0	$ 0