Details of Selected Consolidated Balance Sheet Captions (Details) (USD $)	12 Months Ended
	Sep. 30, 2014		Sep. 30, 2013
Accounts, Notes, Loans and Financing Receivable [Line Items]
Accounts Receivable, Gross, Current	$ 367,392,000		$ 322,616,000
Less: allowance for doubtful accounts	(23,992,000)		(20,624,000)
Net accounts receivable	343,400,000		301,992,000
Schedule of Other Current Assets [Abstract]
Assets from risk management activities current	45,827,000		11,966,000
Deferred gas cost	20,069,000		15,152,000
Taxes receivable	5,481,000		3,141,000
Prepaid expenses	25,039,000		21,666,000
Materials and supplies	5,704,000		5,511,000
Other	9,145,000		6,765,000
Total other current assets	111,265,000		64,201,000
Property, Plant and Equipment [Line Items]
Property, plant and equipment	8,200,121,000		7,446,272,000
Construction in progress	247,579,000		275,747,000
Total property, plant and equipment and construction in progress	8,447,700,000		7,722,019,000
Less accumulated depreciation and amortization	(1,721,794,000)		(1,691,364,000)
Net property, plant and equipment	6,725,906,000	[1]	6,030,655,000	[1]
Public Utilities, Plant and Equipment, Amount of Acquisition Adjustments and Related Accumulated Depreciation	(76,400,000)		(83,800,000)
Goodwill [Line Items]
Goodwill, Beginning Balance	741,363,000
Goodwill, Other Changes	666,000	[2]
Goodwill, Ending Balance	742,029,000
Schedule of Other Noncurrent Assets [Abstract]
Marketable securities	79,613,000		72,682,000
Regulatory assets	230,240,000		273,287,000
Deferred financing costs	13,698,000		15,199,000
Assets from risk management activities noncurrent	13,038,000		109,354,000
Other	14,340,000		14,595,000
Total	350,929,000		485,117,000
Schedule of Accounts Payable and Accrued Liabilities [Abstract]
Trade accounts payable	77,860,000		70,116,000
Accrued gas payable	179,425,000		121,202,000
Accrued liabilities	54,319,000		50,293,000
Total	311,604,000		241,611,000
Schedule of Other Current Liabilities [Abstract]
Customer deposits	82,085,000		76,313,000
Accrued employee costs	46,445,000		54,034,000
Deferred gas costs	35,063,000		16,481,000
Accrued interest	36,768,000		36,744,000
Liabilities from risk management activities current	1,730,000		1,543,000
Taxes payable	77,601,000		66,960,000
Pension and postretirement obligations	11,380,000		22,940,000
Regulatory cost of removal accrual	45,061,000		68,225,000
Current deferred tax liability	48,751,000		14,697,000
Other	17,467,000		10,954,000
Total	402,351,000		368,891,000
Schedule of Other Noncurrent Liabilities [Abstract]
Customer advances for construction	9,883,000		11,723,000
Regulatory liabilities	4,472,000		1,123,000
Asset retirement obligation	10,508,000		6,764,000
Liabilities from risk management activities noncurrent	20,126,000		0
Other	23,881,000		17,953,000
Total	68,870,000		37,563,000
Regulated Distribution Segment [Member]
Schedule of Other Current Assets [Abstract]
Assets from risk management activities current	23,102,000		1,837,000
Property, Plant and Equipment [Line Items]
Net property, plant and equipment	5,202,761,000		4,719,873,000
Goodwill [Line Items]
Goodwill, Beginning Balance	574,190,000
Goodwill, Other Changes	626,000	[2]
Goodwill, Ending Balance	574,816,000
Schedule of Other Noncurrent Assets [Abstract]
Assets from risk management activities noncurrent	13,038,000		109,354,000
Schedule of Other Current Liabilities [Abstract]
Liabilities from risk management activities current	1,730,000		1,543,000
Schedule of Other Noncurrent Liabilities [Abstract]
Liabilities from risk management activities noncurrent	20,126,000		0
Regulated Pipeline 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	1,464,572,000		1,249,767,000
Goodwill [Line Items]
Goodwill, Beginning Balance	132,462,000
Goodwill, Other Changes	40,000	[2]
Goodwill, Ending Balance	132,502,000
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	22,725,000	[3]	10,129,000	[4]
Property, Plant and Equipment [Line Items]
Net property, plant and equipment	58,573,000		61,015,000
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	[3]	0	[4]
Schedule of Other Noncurrent Liabilities [Abstract]
Liabilities from risk management activities noncurrent	0		0
Gas Gathering and Processing Equipment [Member]
Property, Plant and Equipment [Line Items]
Property, plant and equipment	4,821,000		5,020,000
Gas Storage Equipment [Member]
Property, Plant and Equipment [Line Items]
Property, plant and equipment	275,579,000		262,246,000
Gas Transmission Equipment [Member]
Property, Plant and Equipment [Line Items]
Property, plant and equipment	1,622,846,000		1,362,662,000
Gas Distribution Equipment [Member]
Property, Plant and Equipment [Line Items]
Property, plant and equipment	5,522,794,000		5,061,711,000
General Plant [Member]
Property, Plant and Equipment [Line Items]
Property, plant and equipment	735,223,000		716,189,000
Other Capitalized Property Plant and Equipment [Member]
Property, Plant and Equipment [Line Items]
Property, plant and equipment	38,858,000		38,444,000
Trade Accounts Receivable [Member] | Billed Revenues [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Accounts Receivable, Gross, Current	262,937,000		230,712,000
Trade Accounts Receivable [Member] | Unbilled Revenues [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Accounts Receivable, Gross, Current	62,484,000		58,710,000
Other Accounts Receivable [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Accounts Receivable, Gross, Current	$ 41,971,000		$ 33,194,000

[1] Net property, plant and equipment includes plant acquisition adjustments of $(76.4) million and $(83.8) million at September 30, 2014 and 2013.

[2] e annually adjust 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] Includes $25.8 million of cash held on deposit to collateralize certain financial instruments. Of this amount, $3.1 million was used to offset current and noncurent risk management liabilities under master netting arrangements and the remaining $22.7 million is classified as current risk management assets.

[4] Includes $24.8 million of cash held on deposit to collateralize certain financial instruments. Of this amount, $14.7 million was used to offset current and noncurrent risk management liabilities under master netting arrangements and the remaining $10.1 million is classified as current risk management assets.