Details of Selected Consolidated Balance Sheet Captions (Details) - USD ($) $ in Thousands		12 Months Ended
		Sep. 30, 2016		Sep. 30, 2015
Accounts, Notes, Loans and Financing Receivable [Line Items]
Accounts Receivable, Gross, Current		$ 313,374		$ 310,443
Less: allowance for doubtful accounts		(13,367)		(15,283)
Net accounts receivable		300,007		295,160
Schedule of Other Current Assets [Abstract]
Assets from risk management activities current		9,804		9,232
Deferred gas cost		45,184		9,715
Taxes receivable		5,456		4,479
Prepaid expenses		23,053		23,055
Materials and supplies		5,825		12,587
Other		11,507		6,822
Total other current assets		100,829		65,890
Public Utilities, Property, Plant and Equipment [Abstract]
Production plant		66		131
Storage plant		353,523		286,011
Transmission plant		2,232,927		1,844,117
Distribution plant		6,598,990		6,019,001
General plant		761,057		769,311
Intangible plant		40,515		41,131
Property, plant and equipment		9,987,078		8,959,702
Construction in progress		184,062		280,398
Total property, plant and equipment and construction in progress		10,171,140		9,240,100
Less accumulated depreciation and amortization		(1,890,629)		(1,809,520)
Net property, plant and equipment	[1]	8,280,511		7,430,580
Plant acquisition adjustments		(59,800)		(68,100)
Goodwill [Line Items]
Goodwill, Beginning Balance		742,702
Goodwill, Other Changes	[2]	705
Goodwill, Ending Balance		743,407
Schedule of Other Noncurrent Assets [Abstract]
Marketable securities		72,701		74,200
Regulatory assets		214,890		182,573
Assets from risk management activities noncurrent		1,822		368
Other		15,872		18,343
Total		305,285		275,484
Schedule of Accounts Payable and Accrued Liabilities [Abstract]
Trade accounts payable		114,533		78,534
Accrued gas payable		108,526		119,825
Accrued liabilities		36,375		40,583
Total		259,434		238,942
Schedule of Other Current Liabilities [Abstract]
Customer deposits		81,890		100,232
Accrued employee costs		47,058		47,602
Deferred gas costs		20,180		28,100
Accrued interest		34,863		34,914
Liabilities from risk management activities current		56,771		9,568
Taxes payable		104,457		93,674
Pension and postretirement obligations		36,606		21,857
Regulatory cost of removal accrual		52,610		56,123
Current deferred tax liability		0		55,918
Other		14,601		9,966
Total		449,036		457,954
Schedule of Other Noncurrent Liabilities [Abstract]
Customer advances for construction		9,850		9,316
Regulatory liabilities		4,152		3,693
Asset retirement obligation		13,404		9,063
Liabilities from risk management activities noncurrent		184,048		110,539
Other		34,236		29,085
Total		245,690		161,696
Regulated Distribution Segment [Member]
Schedule of Other Current Assets [Abstract]
Assets from risk management activities current		3,029		378
Public Utilities, Property, Plant and Equipment [Abstract]
Net property, plant and equipment		6,220,425		5,670,306
Goodwill [Line Items]
Goodwill, Beginning Balance		575,449
Goodwill, Other Changes	[2]	665
Goodwill, Ending Balance		576,114
Schedule of Other Noncurrent Assets [Abstract]
Assets from risk management activities noncurrent		1,822		368
Schedule of Other Current Liabilities [Abstract]
Liabilities from risk management activities current		56,771		9,568
Schedule of Other Noncurrent Liabilities [Abstract]
Liabilities from risk management activities noncurrent		184,048		110,539
Regulated Pipeline Segment [Member]
Schedule of Other Current Assets [Abstract]
Assets from risk management activities current		0		0
Public Utilities, Property, Plant and Equipment [Abstract]
Net property, plant and equipment		2,008,997		1,706,449
Goodwill [Line Items]
Goodwill, Beginning Balance		132,542
Goodwill, Other Changes	[2]	40
Goodwill, Ending Balance		132,582
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		6,775	[3]	8,854	[4]
Public Utilities, Property, Plant and Equipment [Abstract]
Net property, plant and equipment		51,089		53,825
Goodwill [Line Items]
Goodwill, Beginning Balance		34,711
Goodwill, Other Changes	[2]	0
Goodwill, Ending Balance		34,711
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	[3]	0	[4]
Trade Accounts Receivable [Member] | Billed Revenues [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Accounts Receivable, Gross, Current		206,248		204,585
Trade Accounts Receivable [Member] | Unbilled Revenues [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Accounts Receivable, Gross, Current		67,396		65,008
Other Accounts Receivable [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Accounts Receivable, Gross, Current		$ 39,730		$ 40,850

[1] Net property, plant and equipment includes plant acquisition adjustments of $(59.8) million and $(68.1) million at September 30, 2016 and 2015.

[2] We 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 for fiscal 2016.

[3] ncludes $24.7 million of cash held on deposit to collateralize certain nonregulated financial instruments. Of this amount, $17.9 million was used to offset current and noncurent risk management liabilities under master netting arrangements and the remaining $6.8 million is classified as current risk management assets.

[4] Includes $43.5 million of cash held on deposit to collateralize certain nonregulated financial instruments. Of this amount, $34.6 million was used to offset current and noncurrent risk management liabilities under master netting arrangements and the remaining $8.9 million is classified as current risk management assets.