Segment Information (Details) $ in Thousands	3 Months Ended
	Dec. 31, 2015 USD ($) segment	Dec. 31, 2014 USD ($)	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	$ 906,221	$ 1,258,765
Purchased gas cost	462,458	835,480
Gross profit	443,763	423,285
Operating expenses
Operation and maintenance	124,848	118,582
Depreciation and amortization	71,239	67,593
Taxes, other than income	51,471	49,385
Total operating expenses	247,558	235,560
Operating income	196,205	187,725
Miscellaneous income (expense)	(1,209)	(1,707)
Interest charges	30,483	29,764
Income (loss) before income taxes	164,513	156,254
Income tax expense (benefit)	61,652	58,659
Net income (loss)	102,861	97,595
Capital expenditures	291,674	261,313
ASSETS
Net property, plant and equipment	7,653,287		$ 7,430,580
Investment in subsidiaries	0		0
Current assets
Cash and cash equivalents	78,903	123,832	28,653	$ 42,258
Assets from risk management activities	18,945		9,232
Other current assets	765,422		593,100
Intercompany receivables	0		0
Total current assets	863,270		630,985
Goodwill	742,702		742,702
Noncurrent assets from risk management activities	96		368
Deferred charges and other assets	295,298		288,310
Total assets	9,554,653		9,092,945
CAPITALIZATION AND LIABILITIES
Shareholders' equity	3,272,109		3,194,797
Long-term debt	2,455,474		2,455,388
Total capitalization	5,727,583		5,650,185
Current liabilities
Short-term debt	763,236		457,927
Liabilities from risk management activities	6,738		9,568
Other current liabilities	745,082		687,328
Intercompany payables	0		0
Total current liabilities	1,515,056		1,154,823
Deferred income taxes	1,441,325		1,411,315
Derivative Liability, Noncurrent	103,337		110,539
Regulatory cost of removal obligation	425,555		427,553
Pension and postretirement liabilities	289,939		287,373
Deferred credits and other liabilities	51,858		51,157
Total shareholders' equity and liabilities	9,554,653		9,092,945
Reportable Subsegments [Member]
Segment Reporting Information Profit Loss [Abstract]
Revenues	906,221	1,258,765
Intersubsegment Eliminations [Member]
Segment Reporting Information Profit Loss [Abstract]
Revenues	0	0
Regulated Distribution Segment [Member]
Segment Reporting Information Profit Loss [Abstract]
Total operating revenues	638,602	846,772
Purchased gas cost	305,141	522,960
Gross profit	333,461	323,812
Operating expenses
Operation and maintenance	91,349	86,985
Depreciation and amortization	57,334	55,086
Taxes, other than income	45,261	43,644
Total operating expenses	193,944	185,715
Operating income	139,517	138,097
Miscellaneous income (expense)	(752)	(1,329)
Interest charges	20,705	21,640
Income (loss) before income taxes	118,060	115,128
Income tax expense (benefit)	44,805	43,741
Net income (loss)	73,255	71,387
Capital expenditures	166,544	166,247
ASSETS
Net property, plant and equipment	5,779,479		5,670,306
Investment in subsidiaries	1,020,629		1,038,670
Current assets
Cash and cash equivalents	57,691		23,863
Assets from risk management activities	716		378
Other current assets	589,257		426,270
Intercompany receivables	943,005		887,713
Total current assets	1,590,669		1,338,224
Goodwill	575,449		575,449
Noncurrent assets from risk management activities	96		368
Deferred charges and other assets	277,662		265,693
Total assets	9,243,984		8,888,710
CAPITALIZATION AND LIABILITIES
Shareholders' equity	3,272,109		3,194,797
Long-term debt	2,455,474		2,455,388
Total capitalization	5,727,583		5,650,185
Current liabilities
Short-term debt	1,017,236		782,927
Liabilities from risk management activities	6,738		9,568
Other current liabilities	625,055		569,273
Intercompany payables	0		0
Total current liabilities	1,649,029		1,361,768
Deferred income taxes	1,008,353		1,008,091
Derivative Liability, Noncurrent	103,337		110,539
Regulatory cost of removal obligation	425,555		427,553
Pension and postretirement liabilities	289,939		287,373
Deferred credits and other liabilities	40,188		43,201
Total shareholders' equity and liabilities	9,243,984		8,888,710
Regulated Distribution Segment [Member] | Reportable Subsegments [Member]
Segment Reporting Information Profit Loss [Abstract]
Revenues	637,167	845,404
Regulated Distribution Segment [Member] | Intersubsegment Eliminations [Member]
Segment Reporting Information Profit Loss [Abstract]
Revenues	1,435	1,368
Regulated Pipeline Segment [Member]
Segment Reporting Information Profit Loss [Abstract]
Total operating revenues	94,677	83,567
Purchased gas cost	0	0
Gross profit	94,677	83,567
Operating expenses
Operation and maintenance	27,088	24,615
Depreciation and amortization	12,770	11,382
Taxes, other than income	5,571	4,865
Total operating expenses	45,429	40,862
Operating income	49,248	42,705
Miscellaneous income (expense)	(429)	(252)
Interest charges	9,147	8,324
Income (loss) before income taxes	39,672	34,129
Income tax expense (benefit)	14,086	12,094
Net income (loss)	25,586	22,035
Capital expenditures	125,283	94,754
ASSETS
Net property, plant and equipment	1,821,114		1,706,449
Investment in subsidiaries	0		0
Current assets
Cash and cash equivalents	0		0
Assets from risk management activities	0		0
Other current assets	20,008		24,628
Intercompany receivables	0		0
Total current assets	20,008		24,628
Goodwill	132,542		132,542
Noncurrent assets from risk management activities	0		0
Deferred charges and other assets	17,095		17,288
Total assets	1,990,759		1,880,907
CAPITALIZATION AND LIABILITIES
Shareholders' equity	602,861		577,275
Long-term debt	0		0
Total capitalization	602,861		577,275
Current liabilities
Short-term debt	0		0
Liabilities from risk management activities	0		0
Other current liabilities	28,197		29,780
Intercompany payables	923,366		867,409
Total current liabilities	951,563		897,189
Deferred income taxes	434,497		406,254
Derivative Liability, Noncurrent	0		0
Regulatory cost of removal obligation	0		0
Pension and postretirement liabilities	0		0
Deferred credits and other liabilities	1,838		189
Total shareholders' equity and liabilities	1,990,759		1,880,907
Regulated Pipeline Segment [Member] | Reportable Subsegments [Member]
Segment Reporting Information Profit Loss [Abstract]
Revenues	23,407	20,551
Regulated Pipeline Segment [Member] | Intersubsegment Eliminations [Member]
Segment Reporting Information Profit Loss [Abstract]
Revenues	71,270	63,016
Nonregulated Segment [Member]
Segment Reporting Information Profit Loss [Abstract]
Total operating revenues	272,524	462,288
Purchased gas cost	256,766	446,249
Gross profit	15,758	16,039
Operating expenses
Operation and maintenance	6,544	7,115
Depreciation and amortization	1,135	1,125
Taxes, other than income	639	876
Total operating expenses	8,318	9,116
Operating income	7,440	6,923
Miscellaneous income (expense)	379	300
Interest charges	1,038	226
Income (loss) before income taxes	6,781	6,997
Income tax expense (benefit)	2,761	2,824
Net income (loss)	4,020	4,173
Capital expenditures	(153)	312
ASSETS
Net property, plant and equipment	52,694		53,825
Investment in subsidiaries	0		(2,096)
Current assets
Cash and cash equivalents	21,212		4,790
Assets from risk management activities	18,229		8,854
Other current assets	420,897		480,503
Intercompany receivables	0		0
Total current assets	460,338		494,147
Goodwill	34,711		34,711
Noncurrent assets from risk management activities	0		0
Deferred charges and other assets	541		5,329
Total assets	548,284		585,916
CAPITALIZATION AND LIABILITIES
Shareholders' equity	417,768		461,395
Long-term debt	0		0
Total capitalization	417,768		461,395
Current liabilities
Short-term debt	0		0
Liabilities from risk management activities	0		0
Other current liabilities	102,570		99,480
Intercompany payables	19,639		20,304
Total current liabilities	122,209		119,784
Deferred income taxes	(1,525)		(3,030)
Derivative Liability, Noncurrent	0		0
Regulatory cost of removal obligation	0		0
Pension and postretirement liabilities	0		0
Deferred credits and other liabilities	9,832		7,767
Total shareholders' equity and liabilities	548,284		585,916
Nonregulated Segment [Member] | Reportable Subsegments [Member]
Segment Reporting Information Profit Loss [Abstract]
Revenues	245,647	392,810
Nonregulated Segment [Member] | Intersubsegment Eliminations [Member]
Segment Reporting Information Profit Loss [Abstract]
Revenues	26,877	69,478
Intersegment Elimination [Member]
Segment Reporting Information Profit Loss [Abstract]
Total operating revenues	(99,582)	(133,862)
Purchased gas cost	(99,449)	(133,729)
Gross profit	(133)	(133)
Operating expenses
Operation and maintenance	(133)	(133)
Depreciation and amortization	0	0
Taxes, other than income	0	0
Total operating expenses	(133)	(133)
Operating income	0	0
Miscellaneous income (expense)	(407)	(426)
Interest charges	(407)	(426)
Income (loss) before income taxes	0	0
Income tax expense (benefit)	0	0
Net income (loss)	0	0
Capital expenditures	0	0
ASSETS
Net property, plant and equipment	0		0
Investment in subsidiaries	(1,020,629)		(1,036,574)
Current assets
Cash and cash equivalents	0		0
Assets from risk management activities	0		0
Other current assets	(264,740)		(338,301)
Intercompany receivables	(943,005)		(887,713)
Total current assets	(1,207,745)		(1,226,014)
Goodwill	0		0
Noncurrent assets from risk management activities	0		0
Deferred charges and other assets	0		0
Total assets	(2,228,374)		(2,262,588)
CAPITALIZATION AND LIABILITIES
Shareholders' equity	(1,020,629)		(1,038,670)
Long-term debt	0		0
Total capitalization	(1,020,629)		(1,038,670)
Current liabilities
Short-term debt	(254,000)		(325,000)
Liabilities from risk management activities	0		0
Other current liabilities	(10,740)		(11,205)
Intercompany payables	(943,005)		(887,713)
Total current liabilities	(1,207,745)		(1,223,918)
Deferred income taxes	0		0
Derivative Liability, Noncurrent	0		0
Regulatory cost of removal obligation	0		0
Pension and postretirement liabilities	0		0
Deferred credits and other liabilities	0		0
Total shareholders' equity and liabilities	(2,228,374)		$ (2,262,588)
Intersegment Elimination [Member] | Reportable Subsegments [Member]
Segment Reporting Information Profit Loss [Abstract]
Revenues	0	0
Intersegment Elimination [Member] | Intersubsegment Eliminations [Member]
Segment Reporting Information Profit Loss [Abstract]
Revenues	$ (99,582)	$ (133,862)