Details of Selected Consolidated Balance Sheet Captions (Details) (USD $)	12 Months Ended
	Sep. 30, 2013		Sep. 30, 2012
Accounts, Notes, Loans and Financing Receivable [Line Items]
Accounts Receivable, Gross, Current	$ 322,616,000		$ 243,951,000
Less: allowance for doubtful accounts	(20,624,000)		(9,425,000)
Net accounts receivable	301,992,000		234,526,000
Schedule of Other Current Assets [Abstract]
Assets from risk management activities current	18,099,000		24,707,000
Deferred gas cost	15,152,000		31,359,000
Taxes receivable	3,141,000		1,291,000
Current deferred tax asset	0		27,091,000
Prepaid expenses	21,666,000		17,114,000
Materials and supplies	5,511,000		5,872,000
Asset held for sale	0		154,571,000	[1]
Other	6,765,000		10,777,000
Total other current assets	70,334,000		272,782,000
Property, Plant and Equipment [Line Items]
Property, plant and equipment	7,446,272,000		6,860,358,000
Construction in progress	275,747,000		274,112,000
Total property, plant and equipment and construction in progress	7,722,019,000		7,134,470,000
Less accumulated depreciation and amortization	(1,691,364,000)		(1,658,866,000)
Net property, plant and equipment	6,030,655,000		5,475,604,000
Public Utilities, Plant and Equipment, Amount of Acquisition Adjustments and Related Accumulated Depreciation	83,800,000		91,500,000
Goodwill [Line Items]
Goodwill, Beginning Balance	740,683,000
Goodwill, Other Changes	680,000	[2]
Goodwill, Ending Balance	741,363,000		740,683,000
Schedule of Other Noncurrent Assets [Abstract]
Marketable securities	72,682,000		64,398,000
Regulatory assets	273,287,000		358,495,000
Deferred financing costs	15,199,000		11,157,000
Assets from risk management activities noncurrent	109,354,000		2,283,000
Other	14,474,000		14,929,000
Total	484,996,000		451,262,000
Schedule of Accounts Payable and Accrued Liabilities [Abstract]
Trade accounts payable	70,116,000		82,531,000
Accrued gas payable	121,202,000		81,658,000
Accrued liabilities	50,293,000		51,040,000
Total	241,611,000		215,229,000
Schedule of Other Current Liabilities [Abstract]
Customer deposits	76,313,000		100,926,000
Accrued employee costs	54,034,000		37,675,000
Deferred gas costs	16,481,000		23,072,000
Accrued interest	36,744,000		34,451,000
Liabilities from risk management activities current	1,543,000		85,381,000
Taxes payable	66,960,000		64,319,000
Pension and postretirement obligations	22,940,000		39,625,000
Regulatory cost of removal accrual	68,225,000		78,525,000
Current deferred tax liability	14,697,000		0
Liabilities held for sale	0		11,573,000
Other	10,954,000		14,118,000
Total	368,891,000		489,665,000
Schedule of Other Noncurrent Liabilities [Abstract]
Customer advances for construction	11,723,000		12,937,000
Regulatory liabilities	1,123,000		5,638,000
Asset retirement obligation	6,764,000		10,394,000
Liabilities from risk management activities noncurrent	6,133,000		9,206,000
Other	17,953,000		12,555,000
Total	43,696,000		50,730,000
Natural Gas Distribution Segment [Member]
Schedule of Other Current Assets [Abstract]
Assets from risk management activities current	1,837,000		6,934,000	[3]
Property, Plant and Equipment [Line Items]
Net property, plant and equipment	4,719,873,000		4,432,017,000
Goodwill [Line Items]
Goodwill, Beginning Balance	573,550,000
Goodwill, Other Changes	640,000	[2]
Goodwill, Ending Balance	574,190,000
Schedule of Other Noncurrent Assets [Abstract]
Assets from risk management activities noncurrent	109,354,000		2,283,000
Schedule of Other Current Liabilities [Abstract]
Liabilities from risk management activities current	1,543,000		85,366,000	[3]
Schedule of Other Noncurrent Liabilities [Abstract]
Liabilities from risk management activities noncurrent	0		0
Regulated Transmission and Storage Segment [Member]
Schedule of Other Current Assets [Abstract]
Assets from risk management activities current	0		0
Property, Plant and Equipment [Line Items]
Net property, plant and equipment	1,249,767,000		979,443,000
Goodwill [Line Items]
Goodwill, Beginning Balance	132,422,000
Goodwill, Other Changes	40,000	[2]
Goodwill, Ending Balance	132,462,000
Schedule of Other Noncurrent Assets [Abstract]
Assets from risk management activities noncurrent	0		0
Schedule of Other Current Liabilities [Abstract]
Liabilities from risk management activities current	0		0
Schedule of Other Noncurrent Liabilities [Abstract]
Liabilities from risk management activities noncurrent	0		0
Nonregulated Segment [Member]
Schedule of Other Current Assets [Abstract]
Assets from risk management activities current	16,262,000	[4]	17,773,000	[5]
Property, Plant and Equipment [Line Items]
Net property, plant and equipment	61,015,000		64,144,000
Goodwill [Line Items]
Goodwill, Beginning Balance	34,711,000
Goodwill, Other Changes	0	[2]
Goodwill, Ending Balance	34,711,000
Schedule of Other Noncurrent Assets [Abstract]
Assets from risk management activities noncurrent	0		0
Schedule of Other Current Liabilities [Abstract]
Liabilities from risk management activities current	0	[4]	15,000	[5]
Schedule of Other Noncurrent Liabilities [Abstract]
Liabilities from risk management activities noncurrent	6,133,000		9,206,000
Gas Gathering and Processing Equipment [Member]
Property, Plant and Equipment [Line Items]
Property, plant and equipment	5,020,000		5,020,000
Gas Storage Equipment [Member]
Property, Plant and Equipment [Line Items]
Property, plant and equipment	262,246,000		232,260,000
Gas Transmission Equipment [Member]
Property, Plant and Equipment [Line Items]
Property, plant and equipment	1,362,662,000		1,185,007,000
Gas Distribution Equipment [Member]
Property, Plant and Equipment [Line Items]
Property, plant and equipment	5,061,711,000		4,680,877,000
General Plant [Member]
Property, Plant and Equipment [Line Items]
Property, plant and equipment	716,189,000		717,568,000
Other Capitalized Property Plant and Equipment [Member]
Property, Plant and Equipment [Line Items]
Property, plant and equipment	38,444,000		39,626,000
Trade Accounts Receivable [Member] | Billed Revenues [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Accounts Receivable, Gross, Current	230,712,000		177,953,000
Trade Accounts Receivable [Member] | Unbilled Revenues [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Accounts Receivable, Gross, Current	58,710,000		42,694,000
Other Accounts Receivable [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Accounts Receivable, Gross, Current	$ 33,194,000		$ 23,304,000

[1] As discussed in Note 16, assets and liabilities related to our Georgia operations were classified as “assets held for sale” in other current assets and liabilities in our consolidated balance sheets at September 30, 2012.

[2] During the preparation of the fiscal 2013 tax provision, we adjusted certain deferred taxes recorded in connection with acquisitions completed in fiscal 2001 and fiscal 2004, which resulted in an increase to goodwill and net deferred tax liabilities of $0.7 million.

[3] The September 30, 2012 amounts are presented net of assets and liabilities held for sale in conjunction with the sale of our Georgia operations. At September 30, 2012, assets and liabilities held for sale included $0.1 million of current assets from risk management activities and $0.3 million of current liabilities from risk management activities.

[4] Includes $24.8 million of cash held on deposit to collateralize certain financial instruments. Of this amount, $8.6 million was used to offset current risk management liabilities under master netting arrangements and the remaining $16.2 million is classified as current risk management assets.

[5] Includes $23.7 million of cash held on deposit to collateralize certain financial instruments. Of this amount, $5.9 million was used to offset current risk management liabilities under master netting arrangements and the remaining $17.8 million is classified as current risk management assets.