Segment Information (Details) $ in Thousands	3 Months Ended		9 Months Ended
	Jun. 30, 2016 USD ($)	Jun. 30, 2015 USD ($)	Jun. 30, 2016 USD ($) segment	Jun. 30, 2015 USD ($)	Sep. 30, 2015 USD ($)	Sep. 30, 2014 USD ($)
Segment Reporting Information [Line Items]
Number of operating segments | segment			3
Segment Reporting Information Profit Loss [Abstract]
Total operating revenues	$ 632,916	$ 686,401	$ 2,671,430	$ 3,485,234
Purchased gas cost	225,605	304,728	1,302,545	2,159,538
Gross profit	407,311	381,673	1,368,885	1,325,696
Operating expenses
Operation and maintenance	137,444	132,447	395,958	384,489
Depreciation and amortization	73,459	68,444	216,670	204,059
Taxes, other than income	59,244	63,175	172,872	181,606
Total operating expenses	270,147	264,066	785,500	770,154
Operating income	137,164	117,607	583,385	555,542
Miscellaneous income (expense)	833	634	(1,061)	(2,634)
Interest charges	27,698	27,955	85,741	85,166
Income (loss) before income taxes	110,299	90,286	496,583	467,742
Income tax expense (benefit)	39,106	34,005	180,719	176,182
Net income (loss)	71,193	56,281	315,864	291,560
Capital expenditures	257,775	225,839	796,008	667,483
ASSETS
Net property, plant and equipment	8,053,547		8,053,547		$ 7,430,580
Investment in subsidiaries	0		0		0
Current assets
Cash and cash equivalents	66,206	43,153	66,206	43,153	28,653	$ 42,258
Assets from risk management activities	7,698		7,698		9,232
Other current assets	575,009		575,009		588,421
Intercompany receivables	0		0		0
Total current assets	648,913		648,913		626,306
Goodwill	742,702		742,702		742,702
Noncurrent assets from risk management activities	1,658		1,658		368
Deferred charges and other assets	280,548		280,548		292,989
Total assets	9,727,368		9,727,368		9,092,945
CAPITALIZATION AND LIABILITIES
Shareholders' equity	3,466,724		3,466,724		3,194,797
Long-term debt	2,205,645		2,205,645		2,455,388
Total capitalization	5,672,369		5,672,369		5,650,185
Current liabilities
Current maturities of long-term debt	250,000		250,000		0
Short-term debt	670,466		670,466		457,927
Liabilities from risk management activities	56,883		56,883		9,568
Other current liabilities	552,451		552,451		687,328
Intercompany payables	0		0		0
Total current liabilities	1,529,800		1,529,800		1,154,823
Deferred income taxes	1,585,500		1,585,500		1,411,315
Derivative Liability, Noncurrent	176,491		176,491		110,539
Regulatory cost of removal obligation	427,332		427,332		427,553
Pension and postretirement liabilities	283,579		283,579		287,373
Deferred credits and other liabilities	52,297		52,297		51,157
Total shareholders' equity and liabilities	9,727,368		9,727,368		9,092,945
Reportable Subsegments [Member]
Segment Reporting Information Profit Loss [Abstract]
Revenues	632,916	686,401	2,671,430	3,485,234
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	414,226	416,794	1,902,513	2,394,179
Purchased gas cost	138,845	149,775	884,529	1,397,113
Gross profit	275,381	267,019	1,017,984	997,066
Operating expenses
Operation and maintenance	100,859	98,552	291,388	288,962
Depreciation and amortization	58,916	55,491	173,913	165,730
Taxes, other than income	52,377	56,176	152,324	162,759
Total operating expenses	212,152	210,219	617,625	617,451
Operating income	63,229	56,800	400,359	379,615
Miscellaneous income (expense)	1,111	1,045	209	(1,221)
Interest charges	18,968	19,961	58,390	60,914
Income (loss) before income taxes	45,372	37,884	342,178	317,480
Income tax expense (benefit)	15,516	15,420	124,755	121,776
Net income (loss)	29,856	22,464	217,423	195,704
Capital expenditures	191,202	170,134	533,826	482,371
ASSETS
Net property, plant and equipment	6,067,548		6,067,548		5,670,306
Investment in subsidiaries	1,007,787		1,007,787		1,038,670
Current assets
Cash and cash equivalents	61,441		61,441		23,863
Assets from risk management activities	3,651		3,651		378
Other current assets	370,444		370,444		421,591
Intercompany receivables	981,651		981,651		887,713
Total current assets	1,417,187		1,417,187		1,333,545
Goodwill	575,449		575,449		575,449
Noncurrent assets from risk management activities	750		750		368
Deferred charges and other assets	258,370		258,370		270,372
Total assets	9,327,091		9,327,091		8,888,710
CAPITALIZATION AND LIABILITIES
Shareholders' equity	3,466,724		3,466,724		3,194,797
Long-term debt	2,205,645		2,205,645		2,455,388
Total capitalization	5,672,369		5,672,369		5,650,185
Current liabilities
Current maturities of long-term debt	250,000		250,000
Short-term debt	870,466		870,466		782,927
Liabilities from risk management activities	56,883		56,883		9,568
Other current liabilities	453,831		453,831		569,273
Intercompany payables	0		0		0
Total current liabilities	1,631,180		1,631,180		1,361,768
Deferred income taxes	1,093,755		1,093,755		1,008,091
Derivative Liability, Noncurrent	176,491		176,491		110,539
Regulatory cost of removal obligation	427,332		427,332		427,553
Pension and postretirement liabilities	283,579		283,579		287,373
Deferred credits and other liabilities	42,385		42,385		43,201
Total shareholders' equity and liabilities	9,327,091		9,327,091		8,888,710
Regulated Distribution Segment [Member] | Reportable Subsegments [Member]
Segment Reporting Information Profit Loss [Abstract]
Revenues	411,982	415,160	1,896,636	2,389,037
Regulated Distribution Segment [Member] | Intersubsegment Eliminations [Member]
Segment Reporting Information Profit Loss [Abstract]
Revenues	2,244	1,634	5,877	5,142
Regulated Pipeline Segment [Member]
Segment Reporting Information Profit Loss [Abstract]
Total operating revenues	109,249	97,008	299,629	272,305
Purchased gas cost	0	0	0	0
Gross profit	109,249	97,008	299,629	272,305
Operating expenses
Operation and maintenance	29,083	26,572	83,302	74,029
Depreciation and amortization	13,409	11,816	39,358	34,945
Taxes, other than income	6,220	6,193	18,529	16,296
Total operating expenses	48,712	44,581	141,189	125,270
Operating income	60,537	52,427	158,440	147,035
Miscellaneous income (expense)	(359)	(211)	(1,164)	(842)
Interest charges	9,002	8,299	27,294	25,014
Income (loss) before income taxes	51,176	43,917	129,982	121,179
Income tax expense (benefit)	18,046	15,349	46,081	42,894
Net income (loss)	33,130	28,568	83,901	78,285
Capital expenditures	66,639	55,914	262,058	185,028
ASSETS
Net property, plant and equipment	1,935,087		1,935,087		1,706,449
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	22,269		22,269		24,628
Intercompany receivables	0		0		0
Total current assets	22,269		22,269		24,628
Goodwill	132,542		132,542		132,542
Noncurrent assets from risk management activities	0		0		0
Deferred charges and other assets	21,976		21,976		17,288
Total assets	2,111,874		2,111,874		1,880,907
CAPITALIZATION AND LIABILITIES
Shareholders' equity	661,175		661,175		577,275
Long-term debt	0		0		0
Total capitalization	661,175		661,175		577,275
Current liabilities
Current maturities of long-term debt	0		0
Short-term debt	0		0		0
Liabilities from risk management activities	0		0		0
Other current liabilities	16,590		16,590		29,780
Intercompany payables	953,683		953,683		867,409
Total current liabilities	970,273		970,273		897,189
Deferred income taxes	480,336		480,336		406,254
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	90		90		189
Total shareholders' equity and liabilities	2,111,874		2,111,874		1,880,907
Regulated Pipeline Segment [Member] | Reportable Subsegments [Member]
Segment Reporting Information Profit Loss [Abstract]
Revenues	28,518	25,859	75,344	70,887
Regulated Pipeline Segment [Member] | Intersubsegment Eliminations [Member]
Segment Reporting Information Profit Loss [Abstract]
Revenues	80,731	71,149	224,285	201,418
Nonregulated Segment [Member]
Segment Reporting Information Profit Loss [Abstract]
Total operating revenues	214,555	278,769	774,474	1,179,379
Purchased gas cost	191,741	260,990	722,803	1,122,655
Gross profit	22,814	17,779	51,671	56,724
Operating expenses
Operation and maintenance	7,635	7,456	21,667	21,897
Depreciation and amortization	1,134	1,137	3,399	3,384
Taxes, other than income	647	806	2,019	2,551
Total operating expenses	9,416	9,399	27,085	27,832
Operating income	13,398	8,380	24,586	28,892
Miscellaneous income (expense)	574	345	1,245	897
Interest charges	221	240	1,408	706
Income (loss) before income taxes	13,751	8,485	24,423	29,083
Income tax expense (benefit)	5,544	3,236	9,883	11,512
Net income (loss)	8,207	5,249	14,540	17,571
Capital expenditures	(66)	(209)	124	84
ASSETS
Net property, plant and equipment	50,912		50,912		53,825
Investment in subsidiaries	0		0		(2,096)
Current assets
Cash and cash equivalents	4,765		4,765		4,790
Assets from risk management activities	4,047		4,047		8,854
Other current assets	391,265		391,265		480,503
Intercompany receivables	0		0		0
Total current assets	400,077		400,077		494,147
Goodwill	34,711		34,711		34,711
Noncurrent assets from risk management activities	908		908		0
Deferred charges and other assets	202		202		5,329
Total assets	486,810		486,810		585,916
CAPITALIZATION AND LIABILITIES
Shareholders' equity	346,612		346,612		461,395
Long-term debt	0		0		0
Total capitalization	346,612		346,612		461,395
Current liabilities
Current maturities of long-term debt	0		0
Short-term debt	0		0		0
Liabilities from risk management activities	0		0		0
Other current liabilities	90,999		90,999		99,480
Intercompany payables	27,968		27,968		20,304
Total current liabilities	118,967		118,967		119,784
Deferred income taxes	11,409		11,409		(3,030)
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	9,822		9,822		7,767
Total shareholders' equity and liabilities	486,810		486,810		585,916
Nonregulated Segment [Member] | Reportable Subsegments [Member]
Segment Reporting Information Profit Loss [Abstract]
Revenues	192,416	245,382	699,450	1,025,310
Nonregulated Segment [Member] | Intersubsegment Eliminations [Member]
Segment Reporting Information Profit Loss [Abstract]
Revenues	22,139	33,387	75,024	154,069
Intersegment Elimination [Member]
Segment Reporting Information Profit Loss [Abstract]
Total operating revenues	(105,114)	(106,170)	(305,186)	(360,629)
Purchased gas cost	(104,981)	(106,037)	(304,787)	(360,230)
Gross profit	(133)	(133)	(399)	(399)
Operating expenses
Operation and maintenance	(133)	(133)	(399)	(399)
Depreciation and amortization	0	0	0	0
Taxes, other than income	0	0	0	0
Total operating expenses	(133)	(133)	(399)	(399)
Operating income	0	0	0	0
Miscellaneous income (expense)	(493)	(545)	(1,351)	(1,468)
Interest charges	(493)	(545)	(1,351)	(1,468)
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	(1,007,787)		(1,007,787)		(1,036,574)
Current assets
Cash and cash equivalents	0		0		0
Assets from risk management activities	0		0		0
Other current assets	(208,969)		(208,969)		(338,301)
Intercompany receivables	(981,651)		(981,651)		(887,713)
Total current assets	(1,190,620)		(1,190,620)		(1,226,014)
Goodwill	0		0		0
Noncurrent assets from risk management activities	0		0		0
Deferred charges and other assets	0		0		0
Total assets	(2,198,407)		(2,198,407)		(2,262,588)
CAPITALIZATION AND LIABILITIES
Shareholders' equity	(1,007,787)		(1,007,787)		(1,038,670)
Long-term debt	0		0		0
Total capitalization	(1,007,787)		(1,007,787)		(1,038,670)
Current liabilities
Current maturities of long-term debt	0		0
Short-term debt	(200,000)		(200,000)		(325,000)
Liabilities from risk management activities	0		0		0
Other current liabilities	(8,969)		(8,969)		(11,205)
Intercompany payables	(981,651)		(981,651)		(887,713)
Total current liabilities	(1,190,620)		(1,190,620)		(1,223,918)
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 shareholders' equity and liabilities	(2,198,407)		(2,198,407)		$ (2,262,588)
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	$ (105,114)	$ (106,170)	$ (305,186)	$ (360,629)