Segment Information (Details) $ in Thousands	3 Months Ended		6 Months Ended
	Mar. 31, 2016 USD ($)	Mar. 31, 2015 USD ($)	Mar. 31, 2016 USD ($) segment	Mar. 31, 2015 USD ($)	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	$ 1,132,293	$ 1,540,068	$ 2,038,514	$ 2,798,833
Purchased gas cost	614,482	1,019,330	1,076,940	1,854,810
Gross profit	517,811	520,738	961,574	944,023
Operating expenses
Operation and maintenance	133,666	133,460	258,514	252,042
Depreciation and amortization	71,972	68,022	143,211	135,615
Taxes, other than income	62,157	69,046	113,628	118,431
Total operating expenses	267,795	270,528	515,353	506,088
Operating income	250,016	250,210	446,221	437,935
Miscellaneous income (expense)	(685)	(1,561)	(1,894)	(3,268)
Interest charges	27,560	27,447	58,043	57,211
Income (loss) before income taxes	221,771	221,202	386,284	377,456
Income tax expense (benefit)	79,961	83,518	141,613	142,177
Net income (loss)	141,810	137,684	244,671	235,279
Capital expenditures	246,559	180,331	538,233	441,644
ASSETS
Net property, plant and equipment	7,839,413		7,839,413		$ 7,430,580
Investment in subsidiaries	0		0		0
Current assets
Cash and cash equivalents	47,918	95,525	47,918	95,525	28,653	$ 42,258
Assets from risk management activities	7,474		7,474		9,232
Other current assets	597,520		597,520		588,421
Intercompany receivables	0		0		0
Total current assets	652,912		652,912		626,306
Goodwill	742,702		742,702		742,702
Noncurrent assets from risk management activities	0		0		368
Deferred charges and other assets	308,899		308,899		292,989
Total assets	9,543,926		9,543,926		9,092,945
CAPITALIZATION AND LIABILITIES
Shareholders' equity	3,344,565		3,344,565		3,194,797
Long-term debt	2,455,559		2,455,559		2,455,388
Total capitalization	5,800,124		5,800,124		5,650,185
Current liabilities
Short-term debt	626,929		626,929		457,927
Liabilities from risk management activities	784		784		9,568
Other current liabilities	599,640		599,640		687,328
Intercompany payables	0		0		0
Total current liabilities	1,227,353		1,227,353		1,154,823
Deferred income taxes	1,557,790		1,557,790		1,411,315
Derivative Liability, Noncurrent	185,057		185,057		110,539
Regulatory cost of removal obligation	426,756		426,756		427,553
Pension and postretirement liabilities	294,377		294,377		287,373
Deferred credits and other liabilities	52,469		52,469		51,157
Total shareholders' equity and liabilities	9,543,926		9,543,926		9,092,945
Reportable Subsegments [Member]
Segment Reporting Information Profit Loss [Abstract]
Revenues	1,132,293	1,540,068	2,038,514	2,798,833
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	849,685	1,130,613	1,488,287	1,977,385
Purchased gas cost	440,543	724,378	745,684	1,247,338
Gross profit	409,142	406,235	742,603	730,047
Operating expenses
Operation and maintenance	99,180	103,425	190,529	190,410
Depreciation and amortization	57,663	55,153	114,997	110,239
Taxes, other than income	54,686	62,939	99,947	106,583
Total operating expenses	211,529	221,517	405,473	407,232
Operating income	197,613	184,718	337,130	322,815
Miscellaneous income (expense)	(150)	(937)	(902)	(2,266)
Interest charges	18,717	19,313	39,422	40,953
Income (loss) before income taxes	178,746	164,468	296,806	279,596
Income tax expense (benefit)	64,434	62,615	109,239	106,356
Net income (loss)	114,312	101,853	187,567	173,240
Capital expenditures	176,080	145,990	342,624	312,237
ASSETS
Net property, plant and equipment	5,902,803		5,902,803		5,670,306
Investment in subsidiaries	948,346		948,346		1,038,670
Current assets
Cash and cash equivalents	38,464		38,464		23,863
Assets from risk management activities	637		637		378
Other current assets	430,759		430,759		421,591
Intercompany receivables	980,055		980,055		887,713
Total current assets	1,449,915		1,449,915		1,333,545
Goodwill	575,449		575,449		575,449
Noncurrent assets from risk management activities	0		0		368
Deferred charges and other assets	290,737		290,737		270,372
Total assets	9,167,250		9,167,250		8,888,710
CAPITALIZATION AND LIABILITIES
Shareholders' equity	3,344,565		3,344,565		3,194,797
Long-term debt	2,455,559		2,455,559		2,455,388
Total capitalization	5,800,124		5,800,124		5,650,185
Current liabilities
Short-term debt	822,929		822,929		782,927
Liabilities from risk management activities	784		784		9,568
Other current liabilities	492,645		492,645		569,273
Intercompany payables	0		0		0
Total current liabilities	1,316,358		1,316,358		1,361,768
Deferred income taxes	1,102,679		1,102,679		1,008,091
Derivative Liability, Noncurrent	185,057		185,057		110,539
Regulatory cost of removal obligation	426,756		426,756		427,553
Pension and postretirement liabilities	294,377		294,377		287,373
Deferred credits and other liabilities	41,899		41,899		43,201
Total shareholders' equity and liabilities	9,167,250		9,167,250		8,888,710
Regulated Distribution Segment [Member] | Reportable Subsegments [Member]
Segment Reporting Information Profit Loss [Abstract]
Revenues	847,487	1,128,473	1,484,654	1,973,877
Regulated Distribution Segment [Member] | Intersubsegment Eliminations [Member]
Segment Reporting Information Profit Loss [Abstract]
Revenues	2,198	2,140	3,633	3,508
Regulated Pipeline Segment [Member]
Segment Reporting Information Profit Loss [Abstract]
Total operating revenues	95,703	91,730	190,380	175,297
Purchased gas cost	0	0	0	0
Gross profit	95,703	91,730	190,380	175,297
Operating expenses
Operation and maintenance	27,131	22,842	54,219	47,457
Depreciation and amortization	13,179	11,747	25,949	23,129
Taxes, other than income	6,738	5,238	12,309	10,103
Total operating expenses	47,048	39,827	92,477	80,689
Operating income	48,655	51,903	97,903	94,608
Miscellaneous income (expense)	(376)	(379)	(805)	(631)
Interest charges	9,145	8,391	18,292	16,715
Income (loss) before income taxes	39,134	43,133	78,806	77,262
Income tax expense (benefit)	13,949	15,451	28,035	27,545
Net income (loss)	25,185	27,682	50,771	49,717
Capital expenditures	70,136	34,360	195,419	129,114
ASSETS
Net property, plant and equipment	1,884,620		1,884,620		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	17,465		17,465		24,628
Intercompany receivables	0		0		0
Total current assets	17,465		17,465		24,628
Goodwill	132,542		132,542		132,542
Noncurrent assets from risk management activities	0		0		0
Deferred charges and other assets	17,742		17,742		17,288
Total assets	2,052,369		2,052,369		1,880,907
CAPITALIZATION AND LIABILITIES
Shareholders' equity	628,045		628,045		577,275
Long-term debt	0		0		0
Total capitalization	628,045		628,045		577,275
Current liabilities
Short-term debt	0		0		0
Liabilities from risk management activities	0		0		0
Other current liabilities	11,751		11,751		29,780
Intercompany payables	956,552		956,552		867,409
Total current liabilities	968,303		968,303		897,189
Deferred income taxes	455,277		455,277		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	744		744		189
Total shareholders' equity and liabilities	2,052,369		2,052,369		1,880,907
Regulated Pipeline Segment [Member] | Reportable Subsegments [Member]
Segment Reporting Information Profit Loss [Abstract]
Revenues	23,419	24,477	46,826	45,028
Regulated Pipeline Segment [Member] | Intersubsegment Eliminations [Member]
Segment Reporting Information Profit Loss [Abstract]
Revenues	72,284	67,253	143,554	130,269
Nonregulated Segment [Member]
Segment Reporting Information Profit Loss [Abstract]
Total operating revenues	287,395	438,322	559,919	900,610
Purchased gas cost	274,296	415,416	531,062	861,665
Gross profit	13,099	22,906	28,857	38,945
Operating expenses
Operation and maintenance	7,488	7,326	14,032	14,441
Depreciation and amortization	1,130	1,122	2,265	2,247
Taxes, other than income	733	869	1,372	1,745
Total operating expenses	9,351	9,317	17,669	18,433
Operating income	3,748	13,589	11,188	20,512
Miscellaneous income (expense)	292	252	671	552
Interest charges	149	240	1,187	466
Income (loss) before income taxes	3,891	13,601	10,672	20,598
Income tax expense (benefit)	1,578	5,452	4,339	8,276
Net income (loss)	2,313	8,149	6,333	12,322
Capital expenditures	343	(19)	190	293
ASSETS
Net property, plant and equipment	51,990		51,990		53,825
Investment in subsidiaries	0		0		(2,096)
Current assets
Cash and cash equivalents	9,454		9,454		4,790
Assets from risk management activities	6,837		6,837		8,854
Other current assets	356,348		356,348		480,503
Intercompany receivables	0		0		0
Total current assets	372,639		372,639		494,147
Goodwill	34,711		34,711		34,711
Noncurrent assets from risk management activities	0		0		0
Deferred charges and other assets	420		420		5,329
Total assets	459,760		459,760		585,916
CAPITALIZATION AND LIABILITIES
Shareholders' equity	320,301		320,301		461,395
Long-term debt	0		0		0
Total capitalization	320,301		320,301		461,395
Current liabilities
Short-term debt	0		0		0
Liabilities from risk management activities	0		0		0
Other current liabilities	106,296		106,296		99,480
Intercompany payables	23,503		23,503		20,304
Total current liabilities	129,799		129,799		119,784
Deferred income taxes	(166)		(166)		(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	9,826		9,826		7,767
Total shareholders' equity and liabilities	459,760		459,760		585,916
Nonregulated Segment [Member] | Reportable Subsegments [Member]
Segment Reporting Information Profit Loss [Abstract]
Revenues	261,387	387,118	507,034	779,928
Nonregulated Segment [Member] | Intersubsegment Eliminations [Member]
Segment Reporting Information Profit Loss [Abstract]
Revenues	26,008	51,204	52,885	120,682
Intersegment Elimination [Member]
Segment Reporting Information Profit Loss [Abstract]
Total operating revenues	(100,490)	(120,597)	(200,072)	(254,459)
Purchased gas cost	(100,357)	(120,464)	(199,806)	(254,193)
Gross profit	(133)	(133)	(266)	(266)
Operating expenses
Operation and maintenance	(133)	(133)	(266)	(266)
Depreciation and amortization	0	0	0	0
Taxes, other than income	0	0	0	0
Total operating expenses	(133)	(133)	(266)	(266)
Operating income	0	0	0	0
Miscellaneous income (expense)	(451)	(497)	(858)	(923)
Interest charges	(451)	(497)	(858)	(923)
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	(948,346)		(948,346)		(1,036,574)
Current assets
Cash and cash equivalents	0		0		0
Assets from risk management activities	0		0		0
Other current assets	(207,052)		(207,052)		(338,301)
Intercompany receivables	(980,055)		(980,055)		(887,713)
Total current assets	(1,187,107)		(1,187,107)		(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	(2,135,453)		(2,135,453)		(2,262,588)
CAPITALIZATION AND LIABILITIES
Shareholders' equity	(948,346)		(948,346)		(1,038,670)
Long-term debt	0		0		0
Total capitalization	(948,346)		(948,346)		(1,038,670)
Current liabilities
Short-term debt	(196,000)		(196,000)		(325,000)
Liabilities from risk management activities	0		0		0
Other current liabilities	(11,052)		(11,052)		(11,205)
Intercompany payables	(980,055)		(980,055)		(887,713)
Total current liabilities	(1,187,107)		(1,187,107)		(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	(2,135,453)		(2,135,453)		$ (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	$ (100,490)	$ (120,597)	$ (200,072)	$ (254,459)