Segment Information (Details) $ in Thousands	3 Months Ended		9 Months Ended
	Jun. 30, 2015 USD ($)	Jun. 30, 2014 USD ($)	Jun. 30, 2015 USD ($) segment	Jun. 30, 2014 USD ($)	Sep. 30, 2014 USD ($)	Sep. 30, 2013 USD ($)
Segment Reporting Information [Line Items]
Number of operating segments | segment			3
Segment Reporting Information Profit Loss [Abstract]
Total operating revenues	$ 686,401	$ 943,170	$ 3,485,234	$ 4,151,882
Purchased gas cost	304,728	583,637	2,159,538	2,907,115
Gross profit	381,673	359,533	1,325,696	1,244,767
Operating expenses
Operation and maintenance	132,447	125,559	384,489	365,991
Depreciation and amortization	68,444	63,955	204,059	185,731
Taxes, other than income	63,175	63,414	181,606	165,640
Total operating expenses	264,066	252,928	770,154	717,362
Operating income	117,607	106,605	555,542	527,405
Miscellaneous income (expense)	634	(374)	(2,634)	(4,022)
Interest charges	27,955	31,840	85,166	95,556
Income (loss) before income taxes	90,286	74,391	467,742	427,827
Income tax expense (benefit)	34,005	28,670	176,182	161,723
Net income (loss)	56,281	45,721	291,560	266,104
Capital expenditures	225,839	193,591	667,483	552,600
ASSETS
Net property, plant and equipment	7,212,088		7,212,088		$ 6,725,906
Investment in subsidiaries	0		0		0
Current assets
Cash and cash equivalents	43,153	51,421	43,153	51,421	42,258	$ 66,199
Assets from risk management activities	11,586		11,586		45,827
Other current assets	561,910		561,910		687,755
Intercompany receivables	0		0		0
Total current assets	616,649		616,649		775,840
Goodwill	742,029		742,029		742,029
Noncurrent assets from risk management activities	1,109		1,109		13,038
Deferred charges and other assets	312,614		312,614		337,891
Total Assets	8,884,489		8,884,489		8,594,704
CAPITALIZATION AND LIABILITIES
Shareholders' equity	3,238,255		3,238,255		3,086,232
Long-term debt	2,455,303		2,455,303		2,455,986
Total capitalization	5,693,558		5,693,558		5,542,218
Current liabilities
Short-term debt	251,977		251,977		196,695
Liabilities from risk management activities	4,916		4,916		1,730
Other current liabilities	659,684		659,684		712,225
Intercompany payables	0		0		0
Total current liabilities	916,577		916,577		910,650
Deferred income taxes	1,429,090		1,429,090		1,286,616
Derivative Liability, Noncurrent	47,224		47,224		20,126
Regulatory cost of removal obligation	432,153		432,153		445,387
Pension and postretirement liabilities	318,140		318,140		340,963
Deferred credits and other liabilities	47,747		47,747		48,744
Total Equity and Liabilities	8,884,489		8,884,489		8,594,704
Reportable Subsegments [Member]
Segment Reporting Information Profit Loss [Abstract]
Revenues	686,401	943,170	3,485,234	4,151,882
Intersubsegment Eliminations [Member]
Segment Reporting Information Profit Loss [Abstract]
Revenues	0	0	0	0
Regulated Distribution Segment [Member]
Segment Reporting Information Profit Loss [Abstract]
Total operating revenues	416,794	517,707	2,394,179	2,652,532
Purchased gas cost	149,775	260,042	1,397,113	1,710,508
Gross profit	267,019	257,665	997,066	942,024
Operating expenses
Operation and maintenance	98,552	92,994	288,962	289,433
Depreciation and amortization	55,491	52,542	165,730	152,113
Taxes, other than income	56,176	57,596	162,759	155,286
Total operating expenses	210,219	203,132	617,451	596,832
Operating income	56,800	54,533	379,615	345,192
Miscellaneous income (expense)	1,045	678	(1,221)	304
Interest charges	19,961	23,649	60,914	69,802
Income (loss) before income taxes	37,884	31,562	317,480	275,694
Income tax expense (benefit)	15,420	13,033	121,776	105,665
Net income (loss)	22,464	18,529	195,704	170,029
Capital expenditures	170,134	146,860	482,371	413,921
ASSETS
Net property, plant and equipment	5,543,386		5,543,386		5,202,761
Investment in subsidiaries	1,028,457		1,028,457		952,171
Current assets
Cash and cash equivalents	35,288		35,288		33,303
Assets from risk management activities	780		780		23,102
Other current assets	375,213		375,213		490,408
Intercompany receivables	820,587		820,587		790,442
Total current assets	1,231,868		1,231,868		1,337,255
Goodwill	574,816		574,816		574,816
Noncurrent assets from risk management activities	1,109		1,109		13,038
Deferred charges and other assets	291,740		291,740		309,965
Total Assets	8,671,376		8,671,376		8,390,006
CAPITALIZATION AND LIABILITIES
Shareholders' equity	3,238,255		3,238,255		3,086,232
Long-term debt	2,455,303		2,455,303		2,455,986
Total capitalization	5,693,558		5,693,558		5,542,218
Current liabilities
Short-term debt	570,977		570,977		522,695
Liabilities from risk management activities	4,916		4,916		1,730
Other current liabilities	551,102		551,102		559,765
Intercompany payables	0		0		0
Total current liabilities	1,126,995		1,126,995		1,084,190
Deferred income taxes	1,014,432		1,014,432		913,260
Derivative Liability, Noncurrent	47,224		47,224		20,126
Regulatory cost of removal obligation	432,153		432,153		445,387
Pension and postretirement liabilities	318,140		318,140		340,963
Deferred credits and other liabilities	38,874		38,874		43,862
Total Equity and Liabilities	8,671,376		8,671,376		8,390,006
Regulated Distribution Segment [Member] | Reportable Subsegments [Member]
Segment Reporting Information Profit Loss [Abstract]
Revenues	415,160	516,644	2,389,037	2,648,505
Regulated Distribution Segment [Member] | Intersubsegment Eliminations [Member]
Segment Reporting Information Profit Loss [Abstract]
Revenues	1,634	1,063	5,142	4,027
Regulated Pipeline Segment [Member]
Segment Reporting Information Profit Loss [Abstract]
Total operating revenues	97,008	87,189	272,305	232,145
Purchased gas cost	0	0	0	0
Gross profit	97,008	87,189	272,305	232,145
Operating expenses
Operation and maintenance	26,572	23,570	74,029	57,465
Depreciation and amortization	11,816	10,281	34,945	30,223
Taxes, other than income	6,193	5,054	16,296	8,485
Total operating expenses	44,581	38,905	125,270	96,173
Operating income	52,427	48,284	147,035	135,972
Miscellaneous income (expense)	(211)	(489)	(842)	(2,751)
Interest charges	8,299	9,162	25,014	27,274
Income (loss) before income taxes	43,917	38,633	121,179	105,947
Income tax expense (benefit)	15,349	13,695	42,894	37,454
Net income (loss)	28,568	24,938	78,285	68,493
Capital expenditures	55,914	45,658	185,028	137,579
ASSETS
Net property, plant and equipment	1,613,182		1,613,182		1,464,572
Investment in subsidiaries	0		0		0
Current assets
Cash and cash equivalents	0		0		0
Assets from risk management activities	0		0		0
Other current assets	20,100		20,100		14,009
Intercompany receivables	0		0		0
Total current assets	20,100		20,100		14,009
Goodwill	132,502		132,502		132,502
Noncurrent assets from risk management activities	0		0		0
Deferred charges and other assets	15,305		15,305		21,826
Total Assets	1,781,089		1,781,089		1,632,909
CAPITALIZATION AND LIABILITIES
Shareholders' equity	560,898		560,898		482,612
Long-term debt	0		0		0
Total capitalization	560,898		560,898		482,612
Current liabilities
Short-term debt	0		0		0
Liabilities from risk management activities	0		0		0
Other current liabilities	17,850		17,850		24,790
Intercompany payables	786,493		786,493		763,635
Total current liabilities	804,343		804,343		788,425
Deferred income taxes	415,687		415,687		361,688
Derivative Liability, Noncurrent	0		0		0
Regulatory cost of removal obligation	0		0		0
Pension and postretirement liabilities	0		0		0
Deferred credits and other liabilities	161		161		184
Total Equity and Liabilities	1,781,089		1,781,089		1,632,909
Regulated Pipeline Segment [Member] | Reportable Subsegments [Member]
Segment Reporting Information Profit Loss [Abstract]
Revenues	25,859	24,990	70,887	67,162
Regulated Pipeline Segment [Member] | Intersubsegment Eliminations [Member]
Segment Reporting Information Profit Loss [Abstract]
Revenues	71,149	62,199	201,418	164,983
Nonregulated Segment [Member]
Segment Reporting Information Profit Loss [Abstract]
Total operating revenues	278,769	465,485	1,179,379	1,660,131
Purchased gas cost	260,990	450,672	1,122,655	1,589,163
Gross profit	17,779	14,813	56,724	70,968
Operating expenses
Operation and maintenance	7,456	9,129	21,897	19,463
Depreciation and amortization	1,137	1,132	3,384	3,395
Taxes, other than income	806	764	2,551	1,869
Total operating expenses	9,399	11,025	27,832	24,727
Operating income	8,380	3,788	28,892	46,241
Miscellaneous income (expense)	345	1,018	897	1,785
Interest charges	240	610	706	1,840
Income (loss) before income taxes	8,485	4,196	29,083	46,186
Income tax expense (benefit)	3,236	1,942	11,512	18,604
Net income (loss)	5,249	2,254	17,571	27,582
Capital expenditures	(209)	1,073	84	1,100
ASSETS
Net property, plant and equipment	55,520		55,520		58,573
Investment in subsidiaries	(2,096)		(2,096)		(2,096)
Current assets
Cash and cash equivalents	7,865		7,865		8,955
Assets from risk management activities	10,806		10,806		22,725
Other current assets	497,871		497,871		526,161
Intercompany receivables	0		0		0
Total current assets	516,542		516,542		557,841
Goodwill	34,711		34,711		34,711
Noncurrent assets from risk management activities	0		0		0
Deferred charges and other assets	5,569		5,569		6,100
Total Assets	610,246		610,246		655,129
CAPITALIZATION AND LIABILITIES
Shareholders' equity	467,559		467,559		469,559
Long-term debt	0		0		0
Total capitalization	467,559		467,559		469,559
Current liabilities
Short-term debt	0		0		0
Liabilities from risk management activities	0		0		0
Other current liabilities	100,910		100,910		142,397
Intercompany payables	34,094		34,094		26,807
Total current liabilities	135,004		135,004		169,204
Deferred income taxes	(1,029)		(1,029)		11,668
Derivative Liability, Noncurrent	0		0		0
Regulatory cost of removal obligation	0		0		0
Pension and postretirement liabilities	0		0		0
Deferred credits and other liabilities	8,712		8,712		4,698
Total Equity and Liabilities	610,246		610,246		655,129
Nonregulated Segment [Member] | Reportable Subsegments [Member]
Segment Reporting Information Profit Loss [Abstract]
Revenues	245,382	401,536	1,025,310	1,436,215
Nonregulated Segment [Member] | Intersubsegment Eliminations [Member]
Segment Reporting Information Profit Loss [Abstract]
Revenues	33,387	63,949	154,069	223,916
Intersegment Elimination [Member]
Segment Reporting Information Profit Loss [Abstract]
Total operating revenues	(106,170)	(127,211)	(360,629)	(392,926)
Purchased gas cost	(106,037)	(127,077)	(360,230)	(392,556)
Gross profit	(133)	(134)	(399)	(370)
Operating expenses
Operation and maintenance	(133)	(134)	(399)	(370)
Depreciation and amortization	0	0	0	0
Taxes, other than income	0	0	0	0
Total operating expenses	(133)	(134)	(399)	(370)
Operating income	0	0	0	0
Miscellaneous income (expense)	(545)	(1,581)	(1,468)	(3,360)
Interest charges	(545)	(1,581)	(1,468)	(3,360)
Income (loss) before income taxes	0	0	0	0
Income tax expense (benefit)	0	0	0	0
Net income (loss)	0	0	0	0
Capital expenditures	0	0	0	0
ASSETS
Net property, plant and equipment	0		0		0
Investment in subsidiaries	(1,026,361)		(1,026,361)		(950,075)
Current assets
Cash and cash equivalents	0		0		0
Assets from risk management activities	0		0		0
Other current assets	(331,274)		(331,274)		(342,823)
Intercompany receivables	(820,587)		(820,587)		(790,442)
Total current assets	(1,151,861)		(1,151,861)		(1,133,265)
Goodwill	0		0		0
Noncurrent assets from risk management activities	0		0		0
Deferred charges and other assets	0		0		0
Total Assets	(2,178,222)		(2,178,222)		(2,083,340)
CAPITALIZATION AND LIABILITIES
Shareholders' equity	(1,028,457)		(1,028,457)		(952,171)
Long-term debt	0		0		0
Total capitalization	(1,028,457)		(1,028,457)		(952,171)
Current liabilities
Short-term debt	(319,000)		(319,000)		(326,000)
Liabilities from risk management activities	0		0		0
Other current liabilities	(10,178)		(10,178)		(14,727)
Intercompany payables	(820,587)		(820,587)		(790,442)
Total current liabilities	(1,149,765)		(1,149,765)		(1,131,169)
Deferred income taxes	0		0		0
Derivative Liability, Noncurrent	0		0		0
Regulatory cost of removal obligation	0		0		0
Pension and postretirement liabilities	0		0		0
Deferred credits and other liabilities	0		0		0
Total Equity and Liabilities	(2,178,222)		(2,178,222)		$ (2,083,340)
Intersegment Elimination [Member] | Reportable Subsegments [Member]
Segment Reporting Information Profit Loss [Abstract]
Revenues	0	0	0	0
Intersegment Elimination [Member] | Intersubsegment Eliminations [Member]
Segment Reporting Information Profit Loss [Abstract]
Revenues	$ (106,170)	$ (127,211)	$ (360,629)	$ (392,926)