INFORMATION BY OPERATING SEGMENT (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended		6 Months Ended
	Mar. 31, 2013	Mar. 31, 2012	Mar. 31, 2013	Mar. 31, 2012	Sep. 30, 2012
Operating Segment Information [Abstract]
Operating revenues	$ 364,145	$ 298,897	$ 616,079	$ 550,880
Net economic earnings	30,404	25,908	56,205	47,602
Total assets	1,822,119	1,683,222	1,822,119	1,683,222	1,760,152
Reconciliation of Net Economic Earnings to Net Income [Abstract]
Total Net Economic Earnings above	30,404	25,908	56,205	47,602
Add: Unrealized gain (loss) on energy-related derivative contracts, net of tax	(32)	17	(91)	20
Add: Acquisition, divestiture, and restructuring activities, net of tax	(597)	0	(597)	0
Net Income	29,775	25,925	55,517	47,622
Gas Utility [Member]
Operating Segment Information [Abstract]
Operating revenues	363,912	298,623	614,703	549,525
Net economic earnings	30,197	25,772	55,538	46,851
Total assets	1,821,372	1,680,948	1,821,372	1,680,948
Reconciliation of Net Economic Earnings to Net Income [Abstract]
Total Net Economic Earnings above	30,197	25,772	55,538	46,851
Other [Member]
Operating Segment Information [Abstract]
Operating revenues	233	274	1,376	1,355
Net economic earnings	207	136	667	751
Total assets	747	2,274	747	2,274
Reconciliation of Net Economic Earnings to Net Income [Abstract]
Total Net Economic Earnings above	207	136	667	751
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