Details of Selected Consolidated Balance Sheet Captions (Details) - USD ($) $ in Thousands		12 Months Ended
		Sep. 30, 2015		Sep. 30, 2014
Accounts, Notes, Loans and Financing Receivable [Line Items]
Accounts Receivable, Gross, Current		$ 310,443		$ 367,392
Less: allowance for doubtful accounts		(15,283)		(23,992)
Net accounts receivable		295,160		343,400
Schedule of Other Current Assets [Abstract]
Assets from risk management activities current		9,232		45,827
Deferred gas cost		9,715		20,069
Taxes receivable		4,479		5,481
Prepaid expenses		27,734		25,039
Materials and supplies		12,587		5,704
Other		6,822		9,145
Total other current assets		70,569		111,265
Public Utilities, Property, Plant and Equipment [Abstract]
Production plant		131		4,821
Storage plant		286,011		275,579
Transmission plant		1,844,117		1,622,846
Distribution plant		6,019,001		5,522,794
General plant		769,311		735,223
Intangible plant		41,131		38,858
Property, plant and equipment		8,959,702		8,200,121
Construction in progress		280,398		247,579
Total property, plant and equipment and construction in progress		9,240,100		8,447,700
Less accumulated depreciation and amortization		(1,809,520)		(1,721,794)
Net property, plant and equipment	[1]	7,430,580		6,725,906
Plant acquisition adjustments		(68,100)		(76,400)
Goodwill [Line Items]
Goodwill, Beginning Balance		742,029
Goodwill, Other Changes	[2]	673
Goodwill, Ending Balance		742,702
Schedule of Other Noncurrent Assets [Abstract]
Marketable securities		74,200		79,613
Regulatory assets		182,573		230,240
Deferred financing costs		17,873		13,698
Assets from risk management activities noncurrent		368		13,038
Other		13,664		14,340
Total		288,678		350,929
Schedule of Accounts Payable and Accrued Liabilities [Abstract]
Trade accounts payable		78,534		77,860
Accrued gas payable		119,825		179,425
Accrued liabilities		40,583		50,801
Total		238,942		308,086
Schedule of Other Current Liabilities [Abstract]
Customer deposits		100,232		82,085
Accrued employee costs		47,602		46,445
Deferred gas costs		28,100		35,063
Accrued interest		34,914		36,768
Liabilities from risk management activities current		9,568		1,730
Taxes payable		93,674		77,601
Pension and postretirement obligations		21,857		11,380
Regulatory cost of removal accrual		56,123		45,061
Current deferred tax liability		55,918		48,751
Other		9,966		20,985
Total		457,954		405,869
Schedule of Other Noncurrent Liabilities [Abstract]
Customer advances for construction		9,316		9,883
Regulatory liabilities		3,693		4,472
Asset retirement obligation		9,063		10,508
Liabilities from risk management activities noncurrent		110,539		20,126
Other		29,085		23,881
Total		161,696		68,870
Regulated Distribution Segment [Member]
Schedule of Other Current Assets [Abstract]
Assets from risk management activities current		378		23,102
Public Utilities, Property, Plant and Equipment [Abstract]
Net property, plant and equipment		5,670,306		5,202,761
Goodwill [Line Items]
Goodwill, Beginning Balance		574,816
Goodwill, Other Changes	[2]	633
Goodwill, Ending Balance		575,449
Schedule of Other Noncurrent Assets [Abstract]
Assets from risk management activities noncurrent		368		13,038
Schedule of Other Current Liabilities [Abstract]
Liabilities from risk management activities current		9,568		1,730
Schedule of Other Noncurrent Liabilities [Abstract]
Liabilities from risk management activities noncurrent		110,539		20,126
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		1,706,449		1,464,572
Goodwill [Line Items]
Goodwill, Beginning Balance		132,502
Goodwill, Other Changes	[2]	40
Goodwill, Ending Balance		132,542
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		8,854	[3]	22,725	[4]
Public Utilities, Property, Plant and Equipment [Abstract]
Net property, plant and equipment		53,825		58,573
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		204,585		262,937
Trade Accounts Receivable [Member] | Unbilled Revenues [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Accounts Receivable, Gross, Current		65,008		62,484
Other Accounts Receivable [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Accounts Receivable, Gross, Current		$ 40,850		$ 41,971

[1] Net property, plant and equipment includes plant acquisition adjustments of $(68.1) million and $(76.4) million at September 30, 2015 and 2014.

[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 2015.

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

[4] 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 noncurrent risk management liabilities under master netting arrangements and the remaining $22.7 million is classified as current risk management assets.