Segment Information (Details) customers in Millions		3 Months Ended										12 Months Ended
		Sep. 30, 2016 USD ($) state customers regulated_gas_distributions_divisions		Jun. 30, 2016 USD ($)	Mar. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Sep. 30, 2015 USD ($)		Jun. 30, 2015 USD ($)	Mar. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)	Sep. 30, 2016 USD ($) segment state customers regulated_gas_distributions_divisions		Sep. 30, 2015 USD ($)		Sep. 30, 2014 USD ($)	Sep. 30, 2013 USD ($)
Segment Reporting Information Profit Loss [Abstract]
Operating revenues		$ 678,519,000		$ 632,916,000	$ 1,132,293,000	$ 906,221,000	$ 656,902,000		$ 686,401,000	$ 1,540,068,000	$ 1,258,765,000	$ 3,349,949,000		$ 4,142,136,000		$ 4,940,916,000
Purchased gas cost												1,605,053,000		2,462,119,000		3,358,490,000
Gross profit		376,011,000		407,311,000	517,811,000	443,763,000	354,321,000		381,673,000	520,738,000	423,285,000	1,744,896,000		1,680,017,000		1,582,426,000
Operating expenses
Operation and maintenance												560,766,000		541,868,000		505,154,000
Depreciation and amortization												293,096,000		274,796,000		253,987,000
Taxes, other than income												223,016,000		231,958,000		211,936,000
Total operating expenses												1,076,878,000		1,048,622,000		971,077,000
Operating income		84,633,000		137,164,000	250,016,000	196,205,000	75,853,000		117,607,000	250,210,000	187,725,000	668,018,000		631,395,000		611,349,000
Miscellaneous income (expense), net												(1,593,000)		(4,389,000)		(5,235,000)
Interest charges												115,948,000		116,241,000		129,295,000
Income (loss) before income taxes												550,477,000		510,765,000		476,819,000
Income tax expense												200,373,000		195,690,000		187,002,000
Net income		34,240,000		71,193,000	141,810,000	102,861,000	23,515,000		56,281,000	137,684,000	97,595,000	350,104,000		315,075,000		289,817,000
Capital expenditures												1,086,950,000		963,621,000		824,441,000
ASSETS
Net property, plant and equipment	[1]	8,280,511,000					7,430,580,000					8,280,511,000		7,430,580,000
Investment in subsidaries		0					0					0		0
Current assets
Cash and cash equivalents		47,534,000					28,653,000					47,534,000		28,653,000		42,258,000	$ 66,199,000
Assets from risk management activities current		9,804,000					9,232,000					9,804,000		9,232,000
Other current assets		624,348,000					588,421,000					624,348,000		588,421,000
Intercompany receivables		0					0					0		0
Total current assets		681,686,000					626,306,000					681,686,000		626,306,000
Goodwill		743,407,000					742,702,000					743,407,000		742,702,000
Assets from risk management activities noncurrent		1,822,000					368,000					1,822,000		368,000
Deferred charges and other assets		303,463,000					275,116,000					303,463,000		275,116,000
Total assets		10,010,889,000					9,075,072,000					10,010,889,000		9,075,072,000
CAPITALIZATION AND LIABILITIES
Shareholders' equity		3,463,059,000					3,194,797,000					3,463,059,000		3,194,797,000		3,086,232,000	$ 2,580,409,000
Long-term debt		2,188,779,000					2,437,515,000					2,188,779,000		2,437,515,000
Total capitalization		5,651,838,000					5,632,312,000					5,651,838,000		5,632,312,000
Current liabilities
Current maturities of long-term debt		250,000,000					0					250,000,000		0
Short-term debt		829,811,000					457,927,000					829,811,000		457,927,000
Liabilities from risk management activities current		56,771,000					9,568,000					56,771,000		9,568,000
Other current liabilities		651,699,000					687,328,000					651,699,000		687,328,000
Intercompany payables		0					0					0		0
Total current liabilities		1,788,281,000					1,154,823,000					1,788,281,000		1,154,823,000
Deferred income taxes		1,603,056,000					1,411,315,000					1,603,056,000		1,411,315,000
Liabilities from risk management activities noncurrent		184,048,000					110,539,000					184,048,000		110,539,000
Regulatory cost of removal obligation		424,281,000					427,553,000					424,281,000		427,553,000
Pension and postretirement liabilities		297,743,000					287,373,000					297,743,000		287,373,000
Deferred credits and other liabilities		61,642,000					51,157,000					61,642,000		51,157,000
Total shareholders' equity and liabilities		$ 10,010,889,000					9,075,072,000					$ 10,010,889,000		9,075,072,000
Number Of Customers, Regulated Distribution | customers		3										3
Number Of Divisions, Regulated Distribution | regulated_gas_distributions_divisions		6										6
Number of States in which Entity Operates | state		8										8
Number of Operating Segments | segment												3
Reportable Subsegments [Member]
Segment Reporting Information Profit Loss [Abstract]
Operating revenues from external parties												$ 3,349,949,000		4,142,136,000		4,940,916,000
Revenue from External Customers by Products and Services Table [Abstract]
Operating revenues from external parties												3,349,949,000		4,142,136,000		4,940,916,000
Intersubsegment Eliminations [Member]
Segment Reporting Information Profit Loss [Abstract]
Operating revenues from external parties												0		0		0
Revenue from External Customers by Products and Services Table [Abstract]
Operating revenues from external parties												0		0		0
Regulated Distribution Segment [Member]
Segment Reporting Information Profit Loss [Abstract]
Operating revenues		$ 389,353,000		414,226,000	849,685,000	638,602,000	369,656,000		416,794,000	1,130,613,000	846,772,000	2,291,866,000		2,763,835,000		3,061,546,000
Purchased gas cost												1,019,061,000		1,526,258,000		1,885,031,000
Gross profit												1,272,805,000		1,237,577,000		1,176,515,000
Operating expenses
Operation and maintenance												404,115,000		388,486,000		387,228,000
Depreciation and amortization												233,036,000		223,048,000		208,376,000
Taxes, other than income												196,070,000		205,894,000		196,343,000
Total operating expenses												833,221,000		817,428,000		791,947,000
Operating income												439,584,000		420,149,000		384,568,000
Miscellaneous income (expense), net												455,000		(377,000)		(381,000)
Interest charges												79,404,000		84,132,000		94,918,000
Income (loss) before income taxes												360,635,000		335,640,000		289,269,000
Income tax expense												128,265,000		130,827,000		117,684,000
Net income												232,370,000		204,813,000		171,585,000
Capital expenditures												740,039,000		670,575,000		574,372,000
ASSETS
Net property, plant and equipment		6,220,425,000					5,670,306,000					6,220,425,000		5,670,306,000
Investment in subsidaries		1,026,859,000					1,038,670,000					1,026,859,000		1,038,670,000
Current assets
Cash and cash equivalents		21,072,000					23,863,000					21,072,000		23,863,000
Assets from risk management activities current		3,029,000					378,000					3,029,000		378,000
Other current assets		446,868,000					421,591,000					446,868,000		421,591,000
Intercompany receivables		978,093,000					887,713,000					978,093,000		887,713,000
Total current assets		1,449,062,000					1,333,545,000					1,449,062,000		1,333,545,000
Goodwill		576,114,000					575,449,000					576,114,000		575,449,000
Assets from risk management activities noncurrent		1,822,000					368,000					1,822,000		368,000
Deferred charges and other assets		275,496,000					252,499,000					275,496,000		252,499,000
Total assets		9,549,778,000					8,870,837,000					9,549,778,000		8,870,837,000
CAPITALIZATION AND LIABILITIES
Shareholders' equity		3,463,059,000					3,194,797,000					3,463,059,000		3,194,797,000
Long-term debt		2,188,779,000					2,437,515,000					2,188,779,000		2,437,515,000
Total capitalization		5,651,838,000					5,632,312,000					5,651,838,000		5,632,312,000
Current liabilities
Current maturities of long-term debt		250,000,000										250,000,000
Short-term debt		1,026,811,000					782,927,000					1,026,811,000		782,927,000
Liabilities from risk management activities current		56,771,000					9,568,000					56,771,000		9,568,000
Other current liabilities		549,328,000					569,273,000					549,328,000		569,273,000
Intercompany payables		0					0					0		0
Total current liabilities		1,882,910,000					1,361,768,000					1,882,910,000		1,361,768,000
Deferred income taxes		1,058,895,000					1,008,091,000					1,058,895,000		1,008,091,000
Liabilities from risk management activities noncurrent		184,048,000					110,539,000					184,048,000		110,539,000
Regulatory cost of removal obligation		424,281,000					427,553,000					424,281,000		427,553,000
Pension and postretirement liabilities		297,743,000					287,373,000					297,743,000		287,373,000
Deferred credits and other liabilities		50,063,000					43,201,000					50,063,000		43,201,000
Total shareholders' equity and liabilities		9,549,778,000					8,870,837,000					9,549,778,000		8,870,837,000
Regulated Distribution Segment [Member] | Reportable Subsegments [Member]
Segment Reporting Information Profit Loss [Abstract]
Operating revenues from external parties												2,284,185,000		2,757,585,000		3,056,212,000
Revenue from External Customers by Products and Services Table [Abstract]
Operating revenues from external parties												2,284,185,000		2,757,585,000		3,056,212,000
Regulated Distribution Segment [Member] | Reportable Subsegments [Member] | Gas Sales Revenues [Member]
Segment Reporting Information Profit Loss [Abstract]
Operating revenues from external parties												2,190,334,000		2,662,258,000		2,964,456,000
Revenue from External Customers by Products and Services Table [Abstract]
Operating revenues from external parties												2,190,334,000		2,662,258,000		2,964,456,000
Regulated Distribution Segment [Member] | Reportable Subsegments [Member] | Gas Sales Revenues [Member] | Residential Customers [Member]
Segment Reporting Information Profit Loss [Abstract]
Operating revenues from external parties												1,477,049,000		1,761,689,000		1,933,099,000
Revenue from External Customers by Products and Services Table [Abstract]
Operating revenues from external parties												1,477,049,000		1,761,689,000		1,933,099,000
Regulated Distribution Segment [Member] | Reportable Subsegments [Member] | Gas Sales Revenues [Member] | Commercial Customers [Member]
Segment Reporting Information Profit Loss [Abstract]
Operating revenues from external parties												619,979,000		772,187,000		876,042,000
Revenue from External Customers by Products and Services Table [Abstract]
Operating revenues from external parties												619,979,000		772,187,000		876,042,000
Regulated Distribution Segment [Member] | Reportable Subsegments [Member] | Gas Sales Revenues [Member] | Industrial Customers [Member]
Segment Reporting Information Profit Loss [Abstract]
Operating revenues from external parties												51,999,000		74,981,000		90,536,000
Revenue from External Customers by Products and Services Table [Abstract]
Operating revenues from external parties												51,999,000		74,981,000		90,536,000
Regulated Distribution Segment [Member] | Reportable Subsegments [Member] | Gas Sales Revenues [Member] | Public Authority and Other Customers [Member]
Segment Reporting Information Profit Loss [Abstract]
Operating revenues from external parties												41,307,000		53,401,000		64,779,000
Revenue from External Customers by Products and Services Table [Abstract]
Operating revenues from external parties												41,307,000		53,401,000		64,779,000
Regulated Distribution Segment [Member] | Reportable Subsegments [Member] | Gas Transportation Revenues [Member]
Segment Reporting Information Profit Loss [Abstract]
Operating revenues from external parties												70,383,000		67,475,000		64,049,000
Revenue from External Customers by Products and Services Table [Abstract]
Operating revenues from external parties												70,383,000		67,475,000		64,049,000
Regulated Distribution Segment [Member] | Reportable Subsegments [Member] | Other Gas Revenues [Member]
Segment Reporting Information Profit Loss [Abstract]
Operating revenues from external parties												23,468,000		27,852,000		27,707,000
Revenue from External Customers by Products and Services Table [Abstract]
Operating revenues from external parties												23,468,000		27,852,000		27,707,000
Regulated Distribution Segment [Member] | Intersubsegment Eliminations [Member]
Segment Reporting Information Profit Loss [Abstract]
Operating revenues from external parties												7,681,000		6,250,000		5,334,000
Revenue from External Customers by Products and Services Table [Abstract]
Operating revenues from external parties												7,681,000		6,250,000		5,334,000
Regulated Pipeline Segment [Member]
Segment Reporting Information Profit Loss [Abstract]
Operating revenues		109,204,000		109,249,000	95,703,000	94,677,000	97,807,000		97,008,000	91,730,000	83,567,000	408,833,000		370,112,000		318,459,000
Purchased gas cost												0		0		0
Gross profit												408,833,000		370,112,000		318,459,000
Operating expenses
Operation and maintenance												129,525,000		118,866,000		91,466,000
Depreciation and amortization												55,576,000		47,236,000		41,031,000
Taxes, other than income												24,298,000		22,743,000		13,143,000
Total operating expenses												209,399,000		188,845,000		145,640,000
Operating income												199,434,000		181,267,000		172,819,000
Miscellaneous income (expense), net												(1,683,000)		(1,243,000)		(3,181,000)
Interest charges												36,574,000		33,151,000		36,280,000
Income (loss) before income taxes												161,177,000		146,873,000		133,358,000
Income tax expense												59,488,000		52,211,000		47,167,000
Net income												101,689,000		94,662,000		86,191,000
Capital expenditures												346,400,000		291,603,000		248,230,000
ASSETS
Net property, plant and equipment		2,008,997,000					1,706,449,000					2,008,997,000		1,706,449,000
Investment in subsidaries		0					0					0		0
Current assets
Cash and cash equivalents		0					0					0		0
Assets from risk management activities current		0					0					0		0
Other current assets		19,204,000					24,628,000					19,204,000		24,628,000
Intercompany receivables		0					0					0		0
Total current assets		19,204,000					24,628,000					19,204,000		24,628,000
Goodwill		132,582,000					132,542,000					132,582,000		132,542,000
Assets from risk management activities noncurrent		0					0					0		0
Deferred charges and other assets		27,631,000					17,288,000					27,631,000		17,288,000
Total assets		2,188,414,000					1,880,907,000					2,188,414,000		1,880,907,000
CAPITALIZATION AND LIABILITIES
Shareholders' equity		678,964,000					577,275,000					678,964,000		577,275,000
Long-term debt		0					0					0		0
Total capitalization		678,964,000					577,275,000					678,964,000		577,275,000
Current liabilities
Current maturities of long-term debt		0										0
Short-term debt		0					0					0		0
Liabilities from risk management activities current		0					0					0		0
Other current liabilities		22,427,000					29,780,000					22,427,000		29,780,000
Intercompany payables		950,215,000					867,409,000					950,215,000		867,409,000
Total current liabilities		972,642,000					897,189,000					972,642,000		897,189,000
Deferred income taxes		536,732,000					406,254,000					536,732,000		406,254,000
Liabilities from risk management activities noncurrent		0					0					0		0
Regulatory cost of removal obligation		0					0					0		0
Pension and postretirement liabilities		0					0					0		0
Deferred credits and other liabilities		76,000					189,000					76,000		189,000
Total shareholders' equity and liabilities		2,188,414,000					1,880,907,000					2,188,414,000		1,880,907,000
Regulated Pipeline Segment [Member] | Reportable Subsegments [Member]
Segment Reporting Information Profit Loss [Abstract]
Operating revenues from external parties												104,007,000		97,662,000		92,166,000
Revenue from External Customers by Products and Services Table [Abstract]
Operating revenues from external parties												104,007,000		97,662,000		92,166,000
Regulated Pipeline Segment [Member] | Intersubsegment Eliminations [Member]
Segment Reporting Information Profit Loss [Abstract]
Operating revenues from external parties												304,826,000		272,450,000		226,293,000
Revenue from External Customers by Products and Services Table [Abstract]
Operating revenues from external parties												304,826,000		272,450,000		226,293,000
Nonregulated Segment [Member]
Segment Reporting Information Profit Loss [Abstract]
Operating revenues		291,889,000		214,555,000	287,395,000	272,524,000	292,830,000		278,769,000	438,322,000	462,288,000	1,066,363,000		1,472,209,000		2,067,292,000
Purchased gas cost												1,002,573,000		1,399,349,000		1,979,337,000
Gross profit												63,790,000		72,860,000		87,955,000
Operating expenses
Operation and maintenance												27,658,000		35,048,000		26,963,000
Depreciation and amortization												4,484,000		4,512,000		4,580,000
Taxes, other than income												2,648,000		3,321,000		2,450,000
Total operating expenses												34,790,000		42,881,000		33,993,000
Operating income												29,000,000		29,979,000		53,962,000
Miscellaneous income (expense), net												1,443,000		(760,000)		2,216,000
Interest charges												1,778,000		967,000		1,986,000
Income (loss) before income taxes												28,665,000		28,252,000		54,192,000
Income tax expense												12,620,000		12,652,000		22,151,000
Net income												16,045,000		15,600,000		32,041,000
Capital expenditures												511,000		1,443,000		1,839,000
ASSETS
Net property, plant and equipment		51,089,000					53,825,000					51,089,000		53,825,000
Investment in subsidaries		0					(2,096,000)					0		(2,096,000)
Current assets
Cash and cash equivalents		26,462,000					4,790,000					26,462,000		4,790,000
Assets from risk management activities current		6,775,000	[2]				8,854,000	[3]				6,775,000	[2]	8,854,000	[3]
Other current assets		367,220,000					480,503,000					367,220,000		480,503,000
Intercompany receivables		0					0					0		0
Total current assets		400,457,000					494,147,000					400,457,000		494,147,000
Goodwill		34,711,000					34,711,000					34,711,000		34,711,000
Assets from risk management activities noncurrent		0					0					0		0
Deferred charges and other assets		336,000					5,329,000					336,000		5,329,000
Total assets		486,593,000					585,916,000					486,593,000		585,916,000
CAPITALIZATION AND LIABILITIES
Shareholders' equity		347,895,000					461,395,000					347,895,000		461,395,000
Long-term debt		0					0					0		0
Total capitalization		347,895,000					461,395,000					347,895,000		461,395,000
Current liabilities
Current maturities of long-term debt		0										0
Short-term debt		0					0					0		0
Liabilities from risk management activities current		0	[2]				0	[3]				0	[2]	0	[3]
Other current liabilities		91,888,000					99,480,000					91,888,000		99,480,000
Intercompany payables		27,878,000					20,304,000					27,878,000		20,304,000
Total current liabilities		119,766,000					119,784,000					119,766,000		119,784,000
Deferred income taxes		7,429,000					(3,030,000)					7,429,000		(3,030,000)
Liabilities from risk management activities noncurrent		0	[2]				0	[3]				0	[2]	0	[3]
Regulatory cost of removal obligation		0					0					0		0
Pension and postretirement liabilities		0					0					0		0
Deferred credits and other liabilities		11,503,000					7,767,000					11,503,000		7,767,000
Total shareholders' equity and liabilities		486,593,000					585,916,000					486,593,000		585,916,000
Nonregulated Segment [Member] | Reportable Subsegments [Member]
Segment Reporting Information Profit Loss [Abstract]
Operating revenues from external parties												961,757,000		1,286,889,000		1,792,538,000
Revenue from External Customers by Products and Services Table [Abstract]
Operating revenues from external parties												961,757,000		1,286,889,000		1,792,538,000
Nonregulated Segment [Member] | Intersubsegment Eliminations [Member]
Segment Reporting Information Profit Loss [Abstract]
Operating revenues from external parties												104,606,000		185,320,000		274,754,000
Revenue from External Customers by Products and Services Table [Abstract]
Operating revenues from external parties												104,606,000		185,320,000		274,754,000
Intersegment Elimination [Member]
Segment Reporting Information Profit Loss [Abstract]
Operating revenues		(111,927,000)		$ (105,114,000)	$ (100,490,000)	$ (99,582,000)	(103,391,000)		$ (106,170,000)	$ (120,597,000)	$ (133,862,000)	(417,113,000)		(464,020,000)		(506,381,000)
Purchased gas cost												(416,581,000)		(463,488,000)		(505,878,000)
Gross profit												(532,000)		(532,000)		(503,000)
Operating expenses
Operation and maintenance												(532,000)		(532,000)		(503,000)
Depreciation and amortization												0		0		0
Taxes, other than income												0		0		0
Total operating expenses												(532,000)		(532,000)		(503,000)
Operating income												0		0		0
Miscellaneous income (expense), net												(1,808,000)		(2,009,000)		(3,889,000)
Interest charges												(1,808,000)		(2,009,000)		(3,889,000)
Income (loss) before income taxes												0		0		0
Income tax expense												0		0		0
Net income												0		0		0
Capital expenditures												0		0		0
ASSETS
Net property, plant and equipment		0					0					0		0
Investment in subsidaries		(1,026,859,000)					(1,036,574,000)					(1,026,859,000)		(1,036,574,000)
Current assets
Cash and cash equivalents		0					0					0		0
Assets from risk management activities current		0					0					0		0
Other current assets		(208,944,000)					(338,301,000)					(208,944,000)		(338,301,000)
Intercompany receivables		(978,093,000)					(887,713,000)					(978,093,000)		(887,713,000)
Total current assets		(1,187,037,000)					(1,226,014,000)					(1,187,037,000)		(1,226,014,000)
Goodwill		0					0					0		0
Assets from risk management activities noncurrent		0					0					0		0
Deferred charges and other assets		0					0					0		0
Total assets		(2,213,896,000)					(2,262,588,000)					(2,213,896,000)		(2,262,588,000)
CAPITALIZATION AND LIABILITIES
Shareholders' equity		(1,026,859,000)					(1,038,670,000)					(1,026,859,000)		(1,038,670,000)
Long-term debt		0					0					0		0
Total capitalization		(1,026,859,000)					(1,038,670,000)					(1,026,859,000)		(1,038,670,000)
Current liabilities
Current maturities of long-term debt		0										0
Short-term debt		(197,000,000)					(325,000,000)					(197,000,000)		(325,000,000)
Liabilities from risk management activities current		0					0					0		0
Other current liabilities		(11,944,000)					(11,205,000)					(11,944,000)		(11,205,000)
Intercompany payables		(978,093,000)					(887,713,000)					(978,093,000)		(887,713,000)
Total current liabilities		(1,187,037,000)					(1,223,918,000)					(1,187,037,000)		(1,223,918,000)
Deferred income taxes		0					0					0		0
Liabilities from risk management activities noncurrent		0					0					0		0
Regulatory cost of removal obligation		0					0					0		0
Pension and postretirement liabilities		0					0					0		0
Deferred credits and other liabilities		0					0					0		0
Total shareholders' equity and liabilities		$ (2,213,896,000)					$ (2,262,588,000)					(2,213,896,000)		(2,262,588,000)
Intersegment Elimination [Member] | Reportable Subsegments [Member]
Segment Reporting Information Profit Loss [Abstract]
Operating revenues from external parties												0		0		0
Revenue from External Customers by Products and Services Table [Abstract]
Operating revenues from external parties												0		0		0
Intersegment Elimination [Member] | Intersubsegment Eliminations [Member]
Segment Reporting Information Profit Loss [Abstract]
Operating revenues from external parties												(417,113,000)		(464,020,000)		(506,381,000)
Revenue from External Customers by Products and Services Table [Abstract]
Operating revenues from external parties												$ (417,113,000)		$ (464,020,000)		$ (506,381,000)

[1] Net property, plant and equipment includes plant acquisition adjustments of $(59.8) million and $(68.1) million at September 30, 2016 and 2015.

[2] ncludes $24.7 million of cash held on deposit to collateralize certain nonregulated financial instruments. Of this amount, $17.9 million was used to offset current and noncurent risk management liabilities under master netting arrangements and the remaining $6.8 million is classified as current risk management assets.

[3] Includes $43.5 million of cash held on deposit to collateralize certain nonregulated financial instruments. Of this amount, $34.6 million was used to offset current and noncurrent risk management liabilities under master netting arrangements and the remaining $8.9 million is classified as current risk management assets.