Segment Information (Details) (USD $)	0 Months Ended	3 Months Ended
	Apr. 02, 2013	Dec. 31, 2013 segment	Dec. 31, 2012	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		$ 1,255,148,000	$ 1,034,155,000
Purchased gas cost		866,191,000	671,793,000
Gross profit		388,957,000	362,362,000
Operating expenses
Operation and maintenance		115,757,000	106,527,000
Depreciation and amortization		60,469,000	59,579,000
Taxes, other than income		42,011,000	41,334,000
Total operating expenses		218,237,000	207,440,000
Operating income		170,720,000	154,922,000
Miscellaneous income (expense)		(2,132,000)	698,000
Interest charges		32,115,000	30,522,000
Income (loss) before income taxes		136,473,000	125,098,000
Income tax expense (benefit)		49,457,000	47,750,000
Income (loss) from continuing operations		87,016,000	77,348,000
Income from discontinued operations, net of tax		0	3,117,000
Gain (loss) on sale of discontinued operations, net of tax	5,300,000
Net income (loss)		87,016,000	80,465,000
Capital expenditures		180,567,000	190,027,000
ASSETS
Net property, plant and equipment		6,152,963,000		6,030,655,000
Investment in subsidiaries		0		0
Current assets
Cash and cash equivalents		194,563,000	124,601,000	66,199,000	64,239,000
Assets from risk management activities		97,935,000		11,966,000
Other current assets		1,007,072,000		598,968,000
Intercompany receivables		0		0
Total current assets		1,299,570,000		677,133,000
Intangible assets		110,000		121,000
Goodwill		741,363,000		741,363,000
Noncurrent assets from risk management activities		48,492,000		109,354,000
Deferred charges and other assets		373,593,000		375,642,000
Total Assets		8,616,091,000		7,934,268,000
CAPITALIZATION AND LIABILITIES
Shareholders' equity		2,661,314,000		2,580,409,000
Long-term debt		1,955,750,000		2,455,671,000
Total capitalization		4,617,064,000		5,036,080,000
Current liabilities
Current maturities of long-term debt		500,000,000		0
Short-term debt		689,795,000		367,984,000
Liabilities from risk management activities		36,000		1,543,000
Other current liabilities		823,670,000		608,959,000
Intercompany payables		0		0
Total current liabilities		2,013,501,000		978,486,000
Deferred income taxes		1,230,052,000		1,164,053,000
Regulatory cost of removal obligation		356,617,000		359,299,000
Pension and postretirement liabilities		359,534,000		358,787,000
Deferred credits and other liabilities		39,323,000		37,563,000
Total Equity and Liabilities		8,616,091,000		7,934,268,000
Reportable Subsegments [Member]
Segment Reporting Information Profit Loss [Abstract]
Revenues from external customers		1,255,148,000	1,034,155,000
Intersubsegment Eliminations [Member]
Segment Reporting Information Profit Loss [Abstract]
Revenues from external customers		0	0
Natural Gas Distribution Segment [Member]
Segment Reporting Information Profit Loss [Abstract]
Total operating revenues		843,865,000	666,787,000
Purchased gas cost		544,694,000	387,156,000
Gross profit		299,171,000	279,631,000
Operating expenses
Operation and maintenance		89,663,000	83,736,000
Depreciation and amortization		49,551,000	50,060,000
Taxes, other than income		37,084,000	36,751,000
Total operating expenses		176,298,000	170,547,000
Operating income		122,873,000	109,084,000
Miscellaneous income (expense)		(471,000)	(131,000)
Interest charges		23,325,000	23,563,000
Income (loss) before income taxes		99,077,000	85,390,000
Income tax expense (benefit)		36,320,000	32,297,000
Income (loss) from continuing operations			53,093,000
Income from discontinued operations, net of tax			3,117,000
Net income (loss)		62,757,000	56,210,000
Capital expenditures		127,506,000	145,871,000
ASSETS
Net property, plant and equipment		4,799,657,000		4,719,873,000
Investment in subsidiaries		863,214,000		831,136,000
Current assets
Cash and cash equivalents		152,058,000		4,237,000
Assets from risk management activities		88,934,000		1,837,000
Other current assets		740,359,000		428,366,000
Intercompany receivables		793,589,000		783,738,000
Total current assets		1,774,940,000		1,218,178,000
Intangible assets		0		0
Goodwill		574,190,000		574,190,000
Noncurrent assets from risk management activities		45,878,000		109,354,000
Deferred charges and other assets		345,075,000		347,687,000
Total Assets		8,402,954,000		7,800,418,000
CAPITALIZATION AND LIABILITIES
Shareholders' equity		2,661,314,000		2,580,409,000
Long-term debt		1,955,750,000		2,455,671,000
Total capitalization		4,617,064,000		5,036,080,000
Current liabilities
Current maturities of long-term debt		500,000,000		0
Short-term debt		972,795,000		645,984,000
Liabilities from risk management activities		36,000		1,543,000
Other current liabilities		645,433,000		491,681,000
Intercompany payables		0		0
Total current liabilities		2,118,264,000		1,139,208,000
Deferred income taxes		916,095,000		871,360,000
Regulatory cost of removal obligation		356,617,000		359,299,000
Pension and postretirement liabilities		359,534,000		358,787,000
Deferred credits and other liabilities		35,380,000		35,684,000
Total Equity and Liabilities		8,402,954,000		7,800,418,000
Natural Gas Distribution Segment [Member] | Reportable Subsegments [Member]
Segment Reporting Information Profit Loss [Abstract]
Revenues from external customers		842,432,000	665,549,000
Natural Gas Distribution Segment [Member] | Intersubsegment Eliminations [Member]
Segment Reporting Information Profit Loss [Abstract]
Revenues from external customers		1,433,000	1,238,000
Regulated Transmission and Storage Segment [Member]
Segment Reporting Information Profit Loss [Abstract]
Total operating revenues		71,341,000	60,681,000
Purchased gas cost		0	0
Gross profit		71,341,000	60,681,000
Operating expenses
Operation and maintenance		17,300,000	16,320,000
Depreciation and amortization		9,786,000	8,390,000
Taxes, other than income		4,663,000	3,949,000
Total operating expenses		31,749,000	28,659,000
Operating income		39,592,000	32,022,000
Miscellaneous income (expense)		(1,181,000)	(127,000)
Interest charges		8,957,000	6,871,000
Income (loss) before income taxes		29,454,000	25,024,000
Income tax expense (benefit)		10,008,000	8,919,000
Income (loss) from continuing operations			16,105,000
Income from discontinued operations, net of tax			0
Net income (loss)		19,446,000	16,105,000
Capital expenditures		52,921,000	43,831,000
ASSETS
Net property, plant and equipment		1,293,093,000		1,249,767,000
Investment in subsidiaries		0		0
Current assets
Cash and cash equivalents		0		0
Assets from risk management activities		0		0
Other current assets		11,184,000		11,709,000
Intercompany receivables		0		0
Total current assets		11,184,000		11,709,000
Intangible assets		0		0
Goodwill		132,462,000		132,462,000
Noncurrent assets from risk management activities		0		0
Deferred charges and other assets		20,960,000		19,227,000
Total Assets		1,457,699,000		1,413,165,000
CAPITALIZATION AND LIABILITIES
Shareholders' equity		415,868,000		396,421,000
Long-term debt		0		0
Total capitalization		415,868,000		396,421,000
Current liabilities
Current maturities of long-term debt		0		0
Short-term debt		0		0
Liabilities from risk management activities		0		0
Other current liabilities		20,429,000		20,288,000
Intercompany payables		719,438,000		712,768,000
Total current liabilities		739,867,000		733,056,000
Deferred income taxes		299,819,000		283,554,000
Regulatory cost of removal obligation		0		0
Pension and postretirement liabilities		0		0
Deferred credits and other liabilities		2,145,000		134,000
Total Equity and Liabilities		1,457,699,000		1,413,165,000
Regulated Transmission and Storage Segment [Member] | Reportable Subsegments [Member]
Segment Reporting Information Profit Loss [Abstract]
Revenues from external customers		21,170,000	18,699,000
Regulated Transmission and Storage Segment [Member] | Intersubsegment Eliminations [Member]
Segment Reporting Information Profit Loss [Abstract]
Revenues from external customers		50,171,000	41,982,000
Nonregulated Segment [Member]
Segment Reporting Information Profit Loss [Abstract]
Total operating revenues		447,721,000	399,894,000
Purchased gas cost		429,155,000	377,435,000
Gross profit		18,566,000	22,459,000
Operating expenses
Operation and maintenance		8,915,000	6,882,000
Depreciation and amortization		1,132,000	1,129,000
Taxes, other than income		264,000	634,000
Total operating expenses		10,311,000	8,645,000
Operating income		8,255,000	13,814,000
Miscellaneous income (expense)		324,000	1,667,000
Interest charges		637,000	797,000
Income (loss) before income taxes		7,942,000	14,684,000
Income tax expense (benefit)		3,129,000	6,534,000
Income (loss) from continuing operations			8,150,000
Income from discontinued operations, net of tax			0
Net income (loss)		4,813,000	8,150,000
Capital expenditures		140,000	325,000
ASSETS
Net property, plant and equipment		60,213,000		61,015,000
Investment in subsidiaries		(2,096,000)		(2,096,000)
Current assets
Cash and cash equivalents		42,505,000		61,962,000
Assets from risk management activities		9,001,000		10,129,000
Other current assets		564,079,000		452,126,000
Intercompany receivables		0		0
Total current assets		615,585,000		524,217,000
Intangible assets		110,000		121,000
Goodwill		34,711,000		34,711,000
Noncurrent assets from risk management activities		2,614,000		0
Deferred charges and other assets		7,558,000		8,728,000
Total Assets		718,695,000		626,696,000
CAPITALIZATION AND LIABILITIES
Shareholders' equity		447,346,000		434,715,000
Long-term debt		0		0
Total capitalization		447,346,000		434,715,000
Current liabilities
Current maturities of long-term debt		0		0
Short-term debt		0		0
Liabilities from risk management activities		0		0
Other current liabilities		181,262,000		110,306,000
Intercompany payables		74,151,000		70,970,000
Total current liabilities		255,413,000		181,276,000
Deferred income taxes		14,138,000		8,960,000
Regulatory cost of removal obligation		0		0
Pension and postretirement liabilities		0		0
Deferred credits and other liabilities		1,798,000		1,745,000
Total Equity and Liabilities		718,695,000		626,696,000
Nonregulated Segment [Member] | Reportable Subsegments [Member]
Segment Reporting Information Profit Loss [Abstract]
Revenues from external customers		391,546,000	349,907,000
Nonregulated Segment [Member] | Intersubsegment Eliminations [Member]
Segment Reporting Information Profit Loss [Abstract]
Revenues from external customers		56,175,000	49,987,000
Intersegment Elimination [Member]
Segment Reporting Information Profit Loss [Abstract]
Total operating revenues		(107,779,000)	(93,207,000)
Purchased gas cost		(107,658,000)	(92,798,000)
Gross profit		(121,000)	(409,000)
Operating expenses
Operation and maintenance		(121,000)	(411,000)
Depreciation and amortization		0	0
Taxes, other than income		0	0
Total operating expenses		(121,000)	(411,000)
Operating income		0	2,000
Miscellaneous income (expense)		(804,000)	(711,000)
Interest charges		(804,000)	(709,000)
Income (loss) before income taxes		0	0
Income tax expense (benefit)		0	0
Income (loss) from continuing operations			0
Income from discontinued operations, net of tax			0
Net income (loss)		0	0
Capital expenditures		0	0
ASSETS
Net property, plant and equipment		0		0
Investment in subsidiaries		(861,118,000)		(829,040,000)
Current assets
Cash and cash equivalents		0		0
Assets from risk management activities		0		0
Other current assets		(308,550,000)		(293,233,000)
Intercompany receivables		(793,589,000)		(783,738,000)
Total current assets		(1,102,139,000)		(1,076,971,000)
Intangible assets		0		0
Goodwill		0		0
Noncurrent assets from risk management activities		0		0
Deferred charges and other assets		0		0
Total Assets		(1,963,257,000)		(1,906,011,000)
CAPITALIZATION AND LIABILITIES
Shareholders' equity		(863,214,000)		(831,136,000)
Long-term debt		0		0
Total capitalization		(863,214,000)		(831,136,000)
Current liabilities
Current maturities of long-term debt		0		0
Short-term debt		(283,000,000)		(278,000,000)
Liabilities from risk management activities		0		0
Other current liabilities		(23,454,000)		(13,316,000)
Intercompany payables		(793,589,000)		(783,738,000)
Total current liabilities		(1,100,043,000)		(1,075,054,000)
Deferred income taxes		0		179,000
Regulatory cost of removal obligation		0		0
Pension and postretirement liabilities		0		0
Deferred credits and other liabilities		0		0
Total Equity and Liabilities		(1,963,257,000)		(1,906,011,000)
Intersegment Elimination [Member] | Reportable Subsegments [Member]
Segment Reporting Information Profit Loss [Abstract]
Revenues from external customers		0	0
Intersegment Elimination [Member] | Intersubsegment Eliminations [Member]
Segment Reporting Information Profit Loss [Abstract]
Revenues from external customers		$ (107,779,000)	$ (93,207,000)