Segment Information (Details) (USD $)	0 Months Ended	3 Months Ended		6 Months Ended
	Apr. 02, 2013	Mar. 31, 2014	Mar. 31, 2013	Mar. 31, 2014 segment	Mar. 31, 2013	Sep. 30, 2013	Sep. 30, 2012
Segment Reporting Information [Line Items]
Number of operating segments				3
Segment Reporting Information Profit Loss [Abstract]
Total operating revenues		$ 1,964,322,000	$ 1,308,996,000	$ 3,219,470,000	$ 2,343,151,000
Purchased gas cost		1,468,045,000	876,245,000	2,334,236,000	1,548,038,000
Gross profit		496,277,000	432,751,000	885,234,000	795,113,000
Operating expenses
Operation and maintenance		124,675,000	111,086,000	240,432,000	217,613,000
Depreciation and amortization		61,307,000	57,180,000	121,776,000	116,759,000
Taxes, other than income		60,215,000	54,307,000	102,226,000	95,641,000
Total operating expenses		246,197,000	222,573,000	464,434,000	430,013,000
Operating income		250,080,000	210,178,000	420,800,000	365,100,000
Miscellaneous income (expense)		(1,516,000)	1,712,000	(3,648,000)	2,410,000
Interest charges		31,601,000	33,331,000	63,716,000	63,853,000
Income (loss) before income taxes		216,963,000	178,559,000	353,436,000	303,657,000
Income tax expense (benefit)		83,596,000	66,219,000	133,053,000	113,969,000
Income (loss) from continuing operations		133,367,000	112,340,000	220,383,000	189,688,000
Income from discontinued operations, net of tax		0	4,085,000	0	7,202,000
Gain (loss) on sale of discontinued operations, net of tax	5,300,000
Net income (loss)		133,367,000	116,425,000	220,383,000	196,890,000
Capital expenditures		178,442,000	199,090,000	359,009,000	389,117,000
ASSETS
Net property, plant and equipment		6,269,983,000		6,269,983,000		6,030,655,000
Investment in subsidiaries		0		0		0
Current assets
Cash and cash equivalents		136,740,000	65,547,000	136,740,000	65,547,000	66,199,000	64,239,000
Assets from risk management activities		66,686,000		66,686,000		11,966,000
Other current assets		855,735,000		855,735,000		598,968,000
Intercompany receivables		0		0		0
Total current assets		1,059,161,000		1,059,161,000		677,133,000
Goodwill		741,363,000		741,363,000		741,363,000
Noncurrent assets from risk management activities		39,575,000		39,575,000		109,354,000
Deferred charges and other assets		377,534,000		377,534,000		375,763,000
Total Assets		8,487,616,000		8,487,616,000		7,934,268,000
CAPITALIZATION AND LIABILITIES
Shareholders' equity		3,124,761,000		3,124,761,000		2,580,409,000
Long-term debt		1,955,829,000		1,955,829,000		2,455,671,000
Total capitalization		5,080,590,000		5,080,590,000		5,036,080,000
Current liabilities
Current maturities of long-term debt		500,000,000		500,000,000		0
Short-term debt		0		0		367,984,000
Liabilities from risk management activities						1,543,000
Other current liabilities		863,392,000		863,392,000		608,959,000
Intercompany payables		0		0		0
Total current liabilities		1,363,392,000		1,363,392,000		978,486,000
Deferred income taxes		1,283,551,000		1,283,551,000		1,164,053,000
Regulatory cost of removal obligation		358,262,000		358,262,000		359,299,000
Pension and postretirement liabilities		360,851,000		360,851,000		358,787,000
Deferred credits and other liabilities		40,970,000		40,970,000		37,563,000
Total Equity and Liabilities		8,487,616,000		8,487,616,000		7,934,268,000
Reportable Subsegments [Member]
Segment Reporting Information Profit Loss [Abstract]
Revenues from external customers		1,964,322,000	1,308,996,000	3,219,470,000	2,343,151,000
Intersubsegment Eliminations [Member]
Segment Reporting Information Profit Loss [Abstract]
Revenues from external customers		0	0	0	0
Natural Gas Distribution Segment [Member]
Segment Reporting Information Profit Loss [Abstract]
Total operating revenues		1,290,960,000	905,176,000	2,134,825,000	1,571,963,000
Purchased gas cost		905,772,000	558,170,000	1,450,466,000	945,326,000
Gross profit		385,188,000	347,006,000	684,359,000	626,637,000
Operating expenses
Operation and maintenance		106,776,000	89,344,000	196,439,000	173,080,000
Depreciation and amortization		50,020,000	47,631,000	99,571,000	97,691,000
Taxes, other than income		60,606,000	49,592,000	97,690,000	86,343,000
Total operating expenses		217,402,000	186,567,000	393,700,000	357,114,000
Operating income		167,786,000	160,439,000	290,659,000	269,523,000
Miscellaneous income (expense)		97,000	2,591,000	(374,000)	2,460,000
Interest charges		22,828,000	25,664,000	46,153,000	49,227,000
Income (loss) before income taxes		145,055,000	137,366,000	244,132,000	222,756,000
Income tax expense (benefit)		56,312,000	51,176,000	92,632,000	83,473,000
Income (loss) from continuing operations			86,190,000		139,283,000
Income from discontinued operations, net of tax			4,085,000		7,202,000
Net income (loss)		88,743,000	90,275,000	151,500,000	146,485,000
Capital expenditures		139,555,000	131,465,000	267,061,000	277,336,000
ASSETS
Net property, plant and equipment		4,889,160,000		4,889,160,000		4,719,873,000
Investment in subsidiaries		908,939,000		908,939,000		831,136,000
Current assets
Cash and cash equivalents		73,165,000		73,165,000		4,237,000
Assets from risk management activities		58,746,000		58,746,000		1,837,000
Other current assets		609,806,000		609,806,000		428,366,000
Intercompany receivables		786,428,000		786,428,000		783,738,000
Total current assets		1,528,145,000		1,528,145,000		1,218,178,000
Goodwill		574,190,000		574,190,000		574,190,000
Noncurrent assets from risk management activities		30,665,000		30,665,000		109,354,000
Deferred charges and other assets		350,362,000		350,362,000		347,687,000
Total Assets		8,281,461,000		8,281,461,000		7,800,418,000
CAPITALIZATION AND LIABILITIES
Shareholders' equity		3,124,761,000		3,124,761,000		2,580,409,000
Long-term debt		1,955,829,000		1,955,829,000		2,455,671,000
Total capitalization		5,080,590,000		5,080,590,000		5,036,080,000
Current liabilities
Current maturities of long-term debt		500,000,000		500,000,000		0
Short-term debt		343,000,000		343,000,000		645,984,000
Liabilities from risk management activities						1,543,000
Other current liabilities		658,106,000		658,106,000		491,681,000
Intercompany payables		0		0		0
Total current liabilities		1,501,106,000		1,501,106,000		1,139,208,000
Deferred income taxes		943,831,000		943,831,000		871,360,000
Regulatory cost of removal obligation		358,262,000		358,262,000		359,299,000
Pension and postretirement liabilities		360,851,000		360,851,000		358,787,000
Deferred credits and other liabilities		36,821,000		36,821,000		35,684,000
Total Equity and Liabilities		8,281,461,000		8,281,461,000		7,800,418,000
Natural Gas Distribution Segment [Member] | Reportable Subsegments [Member]
Segment Reporting Information Profit Loss [Abstract]
Revenues from external customers		1,289,429,000	904,181,000	2,131,861,000	1,569,730,000
Natural Gas Distribution Segment [Member] | Intersubsegment Eliminations [Member]
Segment Reporting Information Profit Loss [Abstract]
Revenues from external customers		1,531,000	995,000	2,964,000	2,233,000
Regulated Transmission and Storage Segment [Member]
Segment Reporting Information Profit Loss [Abstract]
Total operating revenues		73,615,000	61,848,000	144,956,000	122,529,000
Purchased gas cost		0	0	0	0
Gross profit		73,615,000	61,848,000	144,956,000	122,529,000
Operating expenses
Operation and maintenance		16,595,000	15,390,000	33,895,000	31,710,000
Depreciation and amortization		10,156,000	8,690,000	19,942,000	17,080,000
Taxes, other than income		(1,232,000)	4,277,000	3,431,000	8,226,000
Total operating expenses		25,519,000	28,357,000	57,268,000	57,016,000
Operating income		48,096,000	33,491,000	87,688,000	65,513,000
Miscellaneous income (expense)		(1,081,000)	(99,000)	(2,262,000)	(226,000)
Interest charges		9,155,000	7,857,000	18,112,000	14,728,000
Income (loss) before income taxes		37,860,000	25,535,000	67,314,000	50,559,000
Income tax expense (benefit)		13,751,000	9,005,000	23,759,000	17,924,000
Income (loss) from continuing operations			16,530,000		32,635,000
Income from discontinued operations, net of tax			0		0
Net income (loss)		24,109,000	16,530,000	43,555,000	32,635,000
Capital expenditures		39,000,000	67,208,000	91,921,000	111,039,000
ASSETS
Net property, plant and equipment		1,322,441,000		1,322,441,000		1,249,767,000
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		14,363,000		14,363,000		11,709,000
Intercompany receivables		0		0		0
Total current assets		14,363,000		14,363,000		11,709,000
Goodwill		132,462,000		132,462,000		132,462,000
Noncurrent assets from risk management activities		0		0		0
Deferred charges and other assets		19,585,000		19,585,000		19,227,000
Total Assets		1,488,851,000		1,488,851,000		1,413,165,000
CAPITALIZATION AND LIABILITIES
Shareholders' equity		439,977,000		439,977,000		396,421,000
Long-term debt		0		0		0
Total capitalization		439,977,000		439,977,000		396,421,000
Current liabilities
Current maturities of long-term debt		0		0		0
Short-term debt		0		0		0
Liabilities from risk management activities						0
Other current liabilities		13,654,000		13,654,000		20,288,000
Intercompany payables		708,046,000		708,046,000		712,768,000
Total current liabilities		721,700,000		721,700,000		733,056,000
Deferred income taxes		324,879,000		324,879,000		283,554,000
Regulatory cost of removal obligation		0		0		0
Pension and postretirement liabilities		0		0		0
Deferred credits and other liabilities		2,295,000		2,295,000		134,000
Total Equity and Liabilities		1,488,851,000		1,488,851,000		1,413,165,000
Regulated Transmission and Storage Segment [Member] | Reportable Subsegments [Member]
Segment Reporting Information Profit Loss [Abstract]
Revenues from external customers		21,002,000	19,655,000	42,172,000	38,354,000
Regulated Transmission and Storage Segment [Member] | Intersubsegment Eliminations [Member]
Segment Reporting Information Profit Loss [Abstract]
Revenues from external customers		52,613,000	42,193,000	102,784,000	84,175,000
Nonregulated Segment [Member]
Segment Reporting Information Profit Loss [Abstract]
Total operating revenues		757,683,000	428,948,000	1,205,404,000	828,842,000
Purchased gas cost		720,094,000	404,641,000	1,149,249,000	782,076,000
Gross profit		37,589,000	24,307,000	56,155,000	46,766,000
Operating expenses
Operation and maintenance		1,419,000	6,763,000	10,334,000	13,645,000
Depreciation and amortization		1,131,000	859,000	2,263,000	1,988,000
Taxes, other than income		841,000	438,000	1,105,000	1,072,000
Total operating expenses		3,391,000	8,060,000	13,702,000	16,705,000
Operating income		34,198,000	16,247,000	42,453,000	30,061,000
Miscellaneous income (expense)		443,000	(91,000)	767,000	1,576,000
Interest charges		593,000	498,000	1,230,000	1,295,000
Income (loss) before income taxes		34,048,000	15,658,000	41,990,000	30,342,000
Income tax expense (benefit)		13,533,000	6,038,000	16,662,000	12,572,000
Income (loss) from continuing operations			9,620,000		17,770,000
Income from discontinued operations, net of tax			0		0
Net income (loss)		20,515,000	9,620,000	25,328,000	17,770,000
Capital expenditures		(113,000)	417,000	27,000	742,000
ASSETS
Net property, plant and equipment		58,382,000		58,382,000		61,015,000
Investment in subsidiaries		(2,096,000)		(2,096,000)		(2,096,000)
Current assets
Cash and cash equivalents		63,575,000		63,575,000		61,962,000
Assets from risk management activities		7,940,000		7,940,000		10,129,000
Other current assets		610,515,000		610,515,000		452,126,000
Intercompany receivables		0		0		0
Total current assets		682,030,000		682,030,000		524,217,000
Goodwill		34,711,000		34,711,000		34,711,000
Noncurrent assets from risk management activities		8,910,000		8,910,000		0
Deferred charges and other assets		7,587,000		7,587,000		8,849,000
Total Assets		789,524,000		789,524,000		626,696,000
CAPITALIZATION AND LIABILITIES
Shareholders' equity		468,962,000		468,962,000		434,715,000
Long-term debt		0		0		0
Total capitalization		468,962,000		468,962,000		434,715,000
Current liabilities
Current maturities of long-term debt		0		0		0
Short-term debt		0		0		0
Liabilities from risk management activities						0
Other current liabilities		225,485,000		225,485,000		110,306,000
Intercompany payables		78,382,000		78,382,000		70,970,000
Total current liabilities		303,867,000		303,867,000		181,276,000
Deferred income taxes		14,841,000		14,841,000		8,960,000
Regulatory cost of removal obligation		0		0		0
Pension and postretirement liabilities		0		0		0
Deferred credits and other liabilities		1,854,000		1,854,000		1,745,000
Total Equity and Liabilities		789,524,000		789,524,000		626,696,000
Nonregulated Segment [Member] | Reportable Subsegments [Member]
Segment Reporting Information Profit Loss [Abstract]
Revenues from external customers		653,891,000	385,160,000	1,045,437,000	735,067,000
Nonregulated Segment [Member] | Intersubsegment Eliminations [Member]
Segment Reporting Information Profit Loss [Abstract]
Revenues from external customers		103,792,000	43,788,000	159,967,000	93,775,000
Intersegment Elimination [Member]
Segment Reporting Information Profit Loss [Abstract]
Total operating revenues		(157,936,000)	(86,976,000)	(265,715,000)	(180,183,000)
Purchased gas cost		(157,821,000)	(86,566,000)	(265,479,000)	(179,364,000)
Gross profit		(115,000)	(410,000)	(236,000)	(819,000)
Operating expenses
Operation and maintenance		(115,000)	(411,000)	(236,000)	(822,000)
Depreciation and amortization		0	0	0	0
Taxes, other than income		0	0	0	0
Total operating expenses		(115,000)	(411,000)	(236,000)	(822,000)
Operating income		0	1,000	0	3,000
Miscellaneous income (expense)		(975,000)	(689,000)	(1,779,000)	(1,400,000)
Interest charges		(975,000)	(688,000)	(1,779,000)	(1,397,000)
Income (loss) before income taxes		0	0	0	0
Income tax expense (benefit)		0	0	0	0
Income (loss) from continuing operations			0		0
Income from discontinued operations, net of tax			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		(906,843,000)		(906,843,000)		(829,040,000)
Current assets
Cash and cash equivalents		0		0		0
Assets from risk management activities		0		0		0
Other current assets		(378,949,000)		(378,949,000)		(293,233,000)
Intercompany receivables		(786,428,000)		(786,428,000)		(783,738,000)
Total current assets		(1,165,377,000)		(1,165,377,000)		(1,076,971,000)
Goodwill		0		0		0
Noncurrent assets from risk management activities		0		0		0
Deferred charges and other assets		0		0		0
Total Assets		(2,072,220,000)		(2,072,220,000)		(1,906,011,000)
CAPITALIZATION AND LIABILITIES
Shareholders' equity		(908,939,000)		(908,939,000)		(831,136,000)
Long-term debt		0		0		0
Total capitalization		(908,939,000)		(908,939,000)		(831,136,000)
Current liabilities
Current maturities of long-term debt		0		0		0
Short-term debt		(343,000,000)		(343,000,000)		(278,000,000)
Liabilities from risk management activities						0
Other current liabilities		(33,853,000)		(33,853,000)		(13,316,000)
Intercompany payables		(786,428,000)		(786,428,000)		(783,738,000)
Total current liabilities		(1,163,281,000)		(1,163,281,000)		(1,075,054,000)
Deferred income taxes		0		0		179,000
Regulatory cost of removal obligation		0		0		0
Pension and postretirement liabilities		0		0		0
Deferred credits and other liabilities		0		0		0
Total Equity and Liabilities		(2,072,220,000)		(2,072,220,000)		(1,906,011,000)
Intersegment Elimination [Member] | Reportable Subsegments [Member]
Segment Reporting Information Profit Loss [Abstract]
Revenues from external customers		0	0	0	0
Intersegment Elimination [Member] | Intersubsegment Eliminations [Member]
Segment Reporting Information Profit Loss [Abstract]
Revenues from external customers		$ (157,936,000)	$ (86,976,000)	$ (265,715,000)	$ (180,183,000)