Discontinued Operations (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended								12 Months Ended
	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2010	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2010
Discontinued Operation Income Loss From Discontinued Operation Disclosures Abstract
Operating revenues									$ 114,703	$ 141,227	$ 128,630
Purchased gas cost									62,902	83,537	77,825
Gross profit									51,801	57,690	50,805
Operating expenses									24,174	27,362	25,202
Operating income									27,627	30,328	25,603
Miscellaneous (expense)									(611)	(57)	31
Income from discontinued operations before income taxes									28,238	30,385	25,572
Income tax expense									10,066	12,372	9,584
Income (loss) from discontinued operations	1,904	3,118	7,027	6,123	1,724	2,584	7,916	5,789	18,172	18,013	15,988
Gain on sale of discountinued operations	6,349	0	0	0					6,349	0	0
Net income (loss) from discontinued operations									24,521	18,013	15,988
Disposal Group Including Discontinued Operation Balance Sheet Disclosures Abstract
Net plant, property & equipment	142,865				127,577				142,865	127,577
Gas stored underground	4,688				11,931				4,688	11,931
Other current assets	6,931				786				6,931	786
Deferred charges and other assets	87				277				87	277
Assets held for sale	154,571				140,571				154,571	140,571
Accounts payable	2,114				1,917				2,114	1,917
Other current liabilities	3,776				4,877				3,776	4,877
Regulatory cost of removal	3,257				10,498				3,257	10,498
Deferred credits and other liabilities	2,426				1,153				2,426	1,153
Liabilities held for sale	11,573				18,445				11,573	18,445
Other Discontinued Operations Disclosures [Abstract]
Discontinued Operation, Gain (Loss) from Disposal of Discontinued Operation, before Income Tax									$ 9,868	$ 0	$ 0
Disposal Group, Including Discontinued Operation, Description and Timing of Disposal									On August 1, 2012, we completed the sale of substantially all of our natural gas distribution assets located in Missouri, Illinois and Iowa to Liberty Energy (Midstates) Corp., an affiliate of Algonquin Power & Utilities Corp. for a cash price of approximately $128 million, pursuant to an asset purchase agreement executed on May 12, 2011. In connection with the sale, we recognized a pre-tax gain of approximately $9.9 million. On August 8, 2012, we entered into a definitive agreement to sell substantially all of our natural gas distribution assets located in Georgia to Liberty Energy (Georgia) Corp., an affiliate of Algonquin Power & Utilities Corp. for a cash price of approximately $141 million. The agreement contains terms and conditions customary for transactions of this type, including typical adjustments to the purchase price at closing, if applicable. The closing of the transaction is subject to the satisfaction of customary conditions including the receipt of applicable regulatory approvals, which we currently anticipate will occur in late fiscal 2013.