Segment Information (Details)	3 Months Ended
	Dec. 31, 2016 USD ($) segment	Dec. 31, 2015 USD ($)	Sep. 30, 2016 USD ($)	Sep. 30, 2015 USD ($)
Segment Reporting Information [Line Items]
Number of operating segments | segment	3
Segment Reporting Information Profit Loss [Abstract]
Total operating revenues	$ 780,168,000	$ 674,753,000
Purchased gas cost	311,305,000	240,326,000
Gross profit	468,863,000	434,427,000
Operating expenses
Operation and maintenance	124,938,000	119,828,000
Depreciation and amortization	76,958,000	70,656,000
Taxes, other than income	57,049,000	51,214,000
Total operating expenses	258,945,000	241,698,000
Operating income	209,918,000	192,729,000
Miscellaneous income (expense)	(994,000)	(879,000)
Interest charges	31,030,000	29,537,000
Income (loss) before income taxes	177,894,000	162,313,000
Income tax expense (benefit)	63,856,000	60,767,000
Income from continuing operations	114,038,000	101,546,000
Income from discontinued operations, net of tax	10,994,000	1,315,000
Net income (loss)	125,032,000	102,861,000
Capital expenditures	297,962,000	290,412,000
ASSETS
Net property, plant and equipment	8,552,962,000		$ 8,268,606,000
Investment in subsidiaries	0		0
Current assets
Cash and cash equivalents	44,624,000	78,903,000	47,534,000	$ 28,653,000
Assets from risk management activities	8,057,000		3,029,000
Current assets held for sale	235,482,000		151,117,000
Other current assets	691,269,000		480,006,000
Intercompany receivables	0		0
Total current assets	979,432,000		681,686,000
Goodwill	729,673,000		726,962,000
Noncurrent assets from risk management activities	1,282,000		1,822,000
Noncurrent assets held for sale	0		28,616,000
Deferred charges and other assets	315,806,000		303,197,000
Total assets	10,579,155,000		10,010,889,000
CAPITALIZATION AND LIABILITIES
Shareholders' equity	3,698,975,000		3,463,059,000
Long-term debt	2,314,199,000		2,188,779,000
Total capitalization	6,013,174,000		5,651,838,000
Current liabilities
Current maturities of long-term debt	250,000,000		250,000,000
Short-term debt	940,747,000		829,811,000
Liabilities from risk management activities	25,060,000		56,771,000
Current liabilities held for sale	109,298,000		72,900,000
Other current liabilities	624,710,000		578,799,000
Intercompany payables	0		0
Total current liabilities	1,949,815,000		1,788,281,000
Deferred income taxes	1,725,433,000		1,603,056,000
Derivative Liability, Noncurrent	97,921,000		184,048,000
Regulatory cost of removal obligation	430,407,000		424,281,000
Pension and postretirement liabilities	301,715,000		297,743,000
Noncurrent liabilities held for sale	0		316,000
Deferred credits and other liabilities	60,690,000		61,326,000
Total shareholders' equity and liabilities	10,579,155,000		10,010,889,000
Reportable Subsegments [Member]
Segment Reporting Information Profit Loss [Abstract]
Revenues	780,168,000	674,753,000
Intersubsegment Eliminations [Member]
Segment Reporting Information Profit Loss [Abstract]
Revenues	0	0
Distribution Segment [Member]
Segment Reporting Information Profit Loss [Abstract]
Total operating revenues	754,656,000	649,443,000
Purchased gas cost	395,346,000	313,991,000
Gross profit	359,310,000	335,452,000
Operating expenses
Operation and maintenance	92,714,000	92,189,000
Depreciation and amortization	61,157,000	57,614,000
Taxes, other than income	50,546,000	45,558,000
Total operating expenses	204,417,000	195,361,000
Operating income	154,893,000	140,091,000
Miscellaneous income (expense)	(633,000)	(477,000)
Interest charges	21,118,000	20,390,000
Income (loss) before income taxes	133,142,000	119,224,000
Income tax expense (benefit)	47,778,000	45,288,000
Income from continuing operations	85,364,000	73,936,000
Income from discontinued operations, net of tax	0	0
Net income (loss)	85,364,000	73,936,000
Capital expenditures	222,484,000	165,407,000
ASSETS
Net property, plant and equipment	6,362,710,000		6,208,465,000
Investment in subsidiaries	834,469,000		768,415,000
Current assets
Cash and cash equivalents	43,733,000		22,117,000
Assets from risk management activities	8,057,000		3,029,000
Current assets held for sale	0		0
Other current assets	666,474,000		486,934,000
Intercompany receivables	1,052,199,000		971,665,000
Total current assets	1,770,463,000		1,483,745,000
Goodwill	586,661,000		583,950,000
Noncurrent assets from risk management activities	1,282,000		1,822,000
Noncurrent assets held for sale			0
Deferred charges and other assets	289,224,000		275,418,000
Total assets	9,844,809,000		9,321,815,000
CAPITALIZATION AND LIABILITIES
Shareholders' equity	3,698,975,000		3,463,059,000
Long-term debt	2,314,199,000		2,188,779,000
Total capitalization	6,013,174,000		5,651,838,000
Current liabilities
Current maturities of long-term debt	250,000,000		250,000,000
Short-term debt	940,747,000		829,811,000
Liabilities from risk management activities	25,060,000		56,771,000
Current liabilities held for sale	0		0
Other current liabilities	602,247,000		549,019,000
Intercompany payables	0		0
Total current liabilities	1,818,054,000		1,685,601,000
Deferred income taxes	1,156,716,000		1,055,348,000
Derivative Liability, Noncurrent	97,921,000		184,048,000
Regulatory cost of removal obligation	407,767,000		397,162,000
Pension and postretirement liabilities	301,715,000		297,743,000
Noncurrent liabilities held for sale			0
Deferred credits and other liabilities	49,462,000		50,075,000
Total shareholders' equity and liabilities	9,844,809,000		9,321,815,000
Distribution Segment [Member] | Reportable Subsegments [Member]
Segment Reporting Information Profit Loss [Abstract]
Revenues	754,266,000	649,113,000
Distribution Segment [Member] | Intersubsegment Eliminations [Member]
Segment Reporting Information Profit Loss [Abstract]
Revenues	390,000	330,000
Pipeline and Storage Segment [Member]
Segment Reporting Information Profit Loss [Abstract]
Total operating revenues	109,952,000	98,416,000
Purchased gas cost	355,000	(559,000)
Gross profit	109,597,000	98,975,000
Operating expenses
Operation and maintenance	32,268,000	27,639,000
Depreciation and amortization	15,801,000	13,042,000
Taxes, other than income	6,503,000	5,656,000
Total operating expenses	54,572,000	46,337,000
Operating income	55,025,000	52,638,000
Miscellaneous income (expense)	(361,000)	(402,000)
Interest charges	9,912,000	9,147,000
Income (loss) before income taxes	44,752,000	43,089,000
Income tax expense (benefit)	16,078,000	15,479,000
Income from continuing operations	28,674,000	27,610,000
Income from discontinued operations, net of tax	0	0
Net income (loss)	28,674,000	27,610,000
Capital expenditures	75,478,000	124,981,000
ASSETS
Net property, plant and equipment	2,190,252,000		2,060,141,000
Investment in subsidiaries	0		0
Current assets
Cash and cash equivalents	0		0
Assets from risk management activities	0		0
Current assets held for sale	0		0
Other current assets	46,009,000		39,078,000
Intercompany receivables	0		0
Total current assets	46,009,000		39,078,000
Goodwill	143,012,000		143,012,000
Noncurrent assets from risk management activities	0		0
Noncurrent assets held for sale			0
Deferred charges and other assets	26,582,000		27,779,000
Total assets	2,405,855,000		2,270,010,000
CAPITALIZATION AND LIABILITIES
Shareholders' equity	731,631,000		701,818,000
Long-term debt	0		0
Total capitalization	731,631,000		701,818,000
Current liabilities
Current maturities of long-term debt	0		0
Short-term debt	0		0
Liabilities from risk management activities	0		1,967,000
Current liabilities held for sale	0		0
Other current liabilities	43,028,000		37,944,000
Intercompany payables	1,048,091,000		957,526,000
Total current liabilities	1,091,119,000		997,437,000
Deferred income taxes	560,401,000		543,390,000
Derivative Liability, Noncurrent	0		169,000
Regulatory cost of removal obligation	22,640,000		27,119,000
Pension and postretirement liabilities	0		0
Noncurrent liabilities held for sale			0
Deferred credits and other liabilities	64,000		77,000
Total shareholders' equity and liabilities	2,405,855,000		2,270,010,000
Pipeline and Storage Segment [Member] | Reportable Subsegments [Member]
Segment Reporting Information Profit Loss [Abstract]
Revenues	25,902,000	25,640,000
Pipeline and Storage Segment [Member] | Intersubsegment Eliminations [Member]
Segment Reporting Information Profit Loss [Abstract]
Revenues	84,050,000	72,776,000
Natural Gas Marketing Segment [Member]
Segment Reporting Information Profit Loss [Abstract]
Total operating revenues	0	0
Purchased gas cost	0	0
Gross profit	0	0
Operating expenses
Operation and maintenance	0	0
Depreciation and amortization	0	0
Taxes, other than income	0	0
Total operating expenses	0	0
Operating income	0	0
Miscellaneous income (expense)	0	0
Interest charges	0	0
Income (loss) before income taxes	0	0
Income tax expense (benefit)	0	0
Income from continuing operations	0	0
Income from discontinued operations, net of tax	10,994,000	1,315,000
Net income (loss)	10,994,000	1,315,000
Capital expenditures	0	24,000
ASSETS
Net property, plant and equipment	0		0
Investment in subsidiaries	0		0
Current assets
Cash and cash equivalents	891,000		25,417,000
Assets from risk management activities	0		0
Current assets held for sale	253,950,000		162,508,000
Other current assets	(6,824,000)		5,000
Intercompany receivables	0		0
Total current assets	248,017,000		187,930,000
Goodwill	0		0
Noncurrent assets from risk management activities	0		0
Noncurrent assets held for sale			28,785,000
Deferred charges and other assets	0		0
Total assets	248,017,000		216,715,000
CAPITALIZATION AND LIABILITIES
Shareholders' equity	102,838,000		66,597,000
Long-term debt	0		0
Total capitalization	102,838,000		66,597,000
Current liabilities
Current maturities of long-term debt	0		0
Short-term debt	0		35,000,000
Liabilities from risk management activities	0		0
Current liabilities held for sale	120,566,000		81,908,000
Other current liabilities	1,025,000		3,263,000
Intercompany payables	4,108,000		14,139,000
Total current liabilities	125,699,000		134,310,000
Deferred income taxes	8,316,000		4,318,000
Derivative Liability, Noncurrent	0		0
Regulatory cost of removal obligation	0		0
Pension and postretirement liabilities	0		0
Noncurrent liabilities held for sale			316,000
Deferred credits and other liabilities	11,164,000		11,174,000
Total shareholders' equity and liabilities	248,017,000		216,715,000
Natural Gas Marketing Segment [Member] | Reportable Subsegments [Member]
Segment Reporting Information Profit Loss [Abstract]
Revenues	0	0
Natural Gas Marketing Segment [Member] | Intersubsegment Eliminations [Member]
Segment Reporting Information Profit Loss [Abstract]
Revenues	0	0
Intersegment Elimination [Member]
Segment Reporting Information Profit Loss [Abstract]
Total operating revenues	(84,440,000)	(73,106,000)
Purchased gas cost	(84,396,000)	(73,106,000)
Gross profit	(44,000)	0
Operating expenses
Operation and maintenance	(44,000)	0
Depreciation and amortization	0	0
Taxes, other than income	0	0
Total operating expenses	(44,000)	0
Operating income	0	0
Miscellaneous income (expense)	0	0
Interest charges	0	0
Income (loss) before income taxes	0	0
Income tax expense (benefit)	0	0
Income from continuing operations	0	0
Income from discontinued operations, net of tax	0	0
Net income (loss)	0	0
Capital expenditures	0	0
ASSETS
Net property, plant and equipment	0		0
Investment in subsidiaries	(834,469,000)		(768,415,000)
Current assets
Cash and cash equivalents	0		0
Assets from risk management activities	0		0
Current assets held for sale	(18,468,000)		(11,391,000)
Other current assets	(14,390,000)		(46,011,000)
Intercompany receivables	(1,052,199,000)		(971,665,000)
Total current assets	(1,085,057,000)		(1,029,067,000)
Goodwill	0		0
Noncurrent assets from risk management activities	0		0
Noncurrent assets held for sale			(169,000)
Deferred charges and other assets	0		0
Total assets	(1,919,526,000)		(1,797,651,000)
CAPITALIZATION AND LIABILITIES
Shareholders' equity	(834,469,000)		(768,415,000)
Long-term debt	0		0
Total capitalization	(834,469,000)		(768,415,000)
Current liabilities
Current maturities of long-term debt	0		0
Short-term debt	0		(35,000,000)
Liabilities from risk management activities	0		(1,967,000)
Current liabilities held for sale	(11,268,000)		(9,008,000)
Other current liabilities	(21,590,000)		(11,427,000)
Intercompany payables	(1,052,199,000)		(971,665,000)
Total current liabilities	(1,085,057,000)		(1,029,067,000)
Deferred income taxes	0		0
Derivative Liability, Noncurrent	0		(169,000)
Regulatory cost of removal obligation	0		0
Pension and postretirement liabilities	0		0
Noncurrent liabilities held for sale			0
Deferred credits and other liabilities	0		0
Total shareholders' equity and liabilities	(1,919,526,000)		$ (1,797,651,000)
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	$ (84,440,000)	$ (73,106,000)