Segment Information (Details)	3 Months Ended		9 Months Ended
	Jun. 30, 2017 USD ($)	Jun. 30, 2016 USD ($)	Jun. 30, 2017 USD ($) segment	Jun. 30, 2016 USD ($)	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	$ 526,501,000	$ 456,212,000	$ 2,294,855,000	$ 2,021,005,000
Purchased gas cost	114,176,000	64,583,000	852,975,000	682,265,000
Operation and maintenance expense	128,690,000	131,388,000	385,867,000	379,073,000
Depreciation and amortization expense	80,023,000	72,880,000	234,648,000	214,927,000
Taxes, other than income, expense	62,948,000	58,965,000	185,611,000	171,959,000
Operating income	140,664,000	128,396,000	635,754,000	572,781,000
Miscellaneous income (expense)	(289,000)	1,118,000	(450,000)	(90,000)
Interest charges	28,498,000	27,679,000	86,472,000	84,775,000
Income (loss) from continuing operations before income taxes	111,877,000	101,835,000	548,832,000	487,916,000
Income tax expense (benefit)	41,069,000	35,692,000	201,974,000	177,224,000
Income from continuing operations	70,808,000	66,143,000	346,858,000	310,692,000
Income from discontinued operations, net of tax	0	5,050,000	13,710,000	5,172,000
Gain (Loss) on Disposal of Discontinued Operation, Net of Tax			2,716,000	0
Net income (loss)	70,808,000	71,193,000	360,568,000	315,864,000
Capital expenditures	252,763,000	253,684,000	812,148,000	789,688,000
Income from discontinued operations, net of tax, before gain on disposal	0	5,050,000	10,994,000	5,172,000
ASSETS
Net property, plant and equipment	8,924,381,000		8,924,381,000		$ 8,268,606,000
Investment in subsidiaries	0		0		0
Current assets
Cash and cash equivalents	69,777,000	66,206,000	69,777,000	66,206,000	47,534,000	$ 28,653,000
Current assets held for sale	0		0		151,117,000
Other current assets	464,605,000		464,605,000		483,035,000
Intercompany receivables	0		0		0
Total current assets	534,382,000		534,382,000		681,686,000
Goodwill	729,673,000		729,673,000		726,962,000
Noncurrent assets held for sale	0		0		28,616,000
Deferred charges and other assets	310,339,000		310,339,000		305,019,000
Total assets	10,498,775,000		10,498,775,000		10,010,889,000
CAPITALIZATION AND LIABILITIES
Shareholders' equity	3,901,710,000		3,901,710,000		3,463,059,000
Long-term debt	3,066,734,000		3,066,734,000		2,188,779,000
Total capitalization	6,968,444,000		6,968,444,000		5,651,838,000
Current liabilities
Current maturities of long-term debt	0		0		250,000,000
Short-term debt	258,573,000		258,573,000		829,811,000
Current liabilities held for sale	0		0		72,900,000
Other current liabilities	487,086,000		487,086,000		635,570,000
Intercompany payables	0		0		0
Total current liabilities	745,659,000		745,659,000		1,788,281,000
Deferred income taxes	1,853,564,000		1,853,564,000		1,603,056,000
Regulatory cost of removal obligation	457,060,000		457,060,000		424,281,000
Pension and postretirement liabilities	304,919,000		304,919,000		297,743,000
Noncurrent liabilities held for sale	0		0		316,000
Deferred credits and other liabilities	169,129,000		169,129,000		245,374,000
Total shareholders' equity and liabilities	10,498,775,000		10,498,775,000		10,010,889,000
Reportable Subsegments [Member]
Segment Reporting Information Profit Loss [Abstract]
Revenues	526,501,000	456,212,000	2,294,855,000	2,021,005,000
Intersubsegment Eliminations [Member]
Segment Reporting Information Profit Loss [Abstract]
Revenues	0	0	0	0
Distribution Segment [Member]
Segment Reporting Information Profit Loss [Abstract]
Total operating revenues	494,060,000	424,905,000	2,211,257,000	1,936,475,000
Purchased gas cost	197,767,000	147,569,000	1,106,209,000	912,231,000
Operation and maintenance expense	99,631,000	101,819,000	296,048,000	294,154,000
Depreciation and amortization expense	62,760,000	59,193,000	185,219,000	174,748,000
Taxes, other than income, expense	56,850,000	52,662,000	165,032,000	153,198,000
Operating income	77,052,000	63,662,000	458,749,000	402,144,000
Miscellaneous income (expense)	(62,000)	1,243,000	334,000	804,000
Interest charges	18,394,000	18,677,000	56,437,000	57,481,000
Income (loss) from continuing operations before income taxes	58,596,000	46,228,000	402,646,000	345,467,000
Income tax expense (benefit)	22,082,000	15,867,000	149,623,000	126,090,000
Income from continuing operations		30,361,000	253,023,000	219,377,000
Gain (Loss) on Disposal of Discontinued Operation, Net of Tax			0
Net income (loss)	36,514,000	30,361,000	253,023,000	219,377,000
Capital expenditures	205,780,000	187,470,000	636,449,000	528,063,000
Income from discontinued operations, net of tax, before gain on disposal		0	0	0
ASSETS
Net property, plant and equipment	6,678,875,000		6,678,875,000		6,208,465,000
Investment in subsidiaries	798,994,000		798,994,000		768,415,000
Current assets
Cash and cash equivalents	69,777,000		69,777,000		22,117,000
Current assets held for sale					0
Other current assets	437,700,000		437,700,000		489,963,000
Intercompany receivables	983,866,000		983,866,000		971,665,000
Total current assets	1,491,343,000		1,491,343,000		1,483,745,000
Goodwill	586,661,000		586,661,000		583,950,000
Noncurrent assets held for sale					0
Deferred charges and other assets	280,240,000		280,240,000		277,240,000
Total assets	9,836,113,000		9,836,113,000		9,321,815,000
CAPITALIZATION AND LIABILITIES
Shareholders' equity	3,901,710,000		3,901,710,000		3,463,059,000
Long-term debt	3,066,734,000		3,066,734,000		2,188,779,000
Total capitalization	6,968,444,000		6,968,444,000		5,651,838,000
Current liabilities
Current maturities of long-term debt					250,000,000
Short-term debt	258,573,000		258,573,000		829,811,000
Current liabilities held for sale					0
Other current liabilities	451,026,000		451,026,000		605,790,000
Intercompany payables	0		0		0
Total current liabilities	709,599,000		709,599,000		1,685,601,000
Deferred income taxes	1,251,528,000		1,251,528,000		1,055,348,000
Regulatory cost of removal obligation	432,531,000		432,531,000		397,162,000
Pension and postretirement liabilities	304,919,000		304,919,000		297,743,000
Noncurrent liabilities held for sale					0
Deferred credits and other liabilities	169,092,000		169,092,000		234,123,000
Total shareholders' equity and liabilities	9,836,113,000		9,836,113,000		9,321,815,000
Distribution Segment [Member] | Reportable Subsegments [Member]
Segment Reporting Information Profit Loss [Abstract]
Revenues	493,738,000	424,553,000	2,210,221,000	1,935,421,000
Distribution Segment [Member] | Intersubsegment Eliminations [Member]
Segment Reporting Information Profit Loss [Abstract]
Revenues	322,000	352,000	1,036,000	1,054,000
Pipeline and Storage Segment [Member]
Segment Reporting Information Profit Loss [Abstract]
Total operating revenues	117,283,000	113,855,000	339,207,000	314,424,000
Purchased gas cost	1,251,000	(438,000)	2,331,000	(72,000)
Operation and maintenance expense	29,059,000	29,569,000	89,863,000	84,919,000
Depreciation and amortization expense	17,263,000	13,687,000	49,429,000	40,179,000
Taxes, other than income, expense	6,098,000	6,303,000	20,579,000	18,761,000
Operating income	63,612,000	64,734,000	177,005,000	170,637,000
Miscellaneous income (expense)	(227,000)	(125,000)	(784,000)	(894,000)
Interest charges	10,104,000	9,002,000	30,035,000	27,294,000
Income (loss) from continuing operations before income taxes	53,281,000	55,607,000	146,186,000	142,449,000
Income tax expense (benefit)	18,987,000	19,825,000	52,351,000	51,134,000
Income from continuing operations		35,782,000	93,835,000	91,315,000
Gain (Loss) on Disposal of Discontinued Operation, Net of Tax			0
Net income (loss)	34,294,000	35,782,000	93,835,000	91,315,000
Capital expenditures	46,983,000	66,108,000	175,699,000	261,446,000
Income from discontinued operations, net of tax, before gain on disposal		0	0	0
ASSETS
Net property, plant and equipment	2,245,506,000		2,245,506,000		2,060,141,000
Investment in subsidiaries	13,851,000		13,851,000		13,854,000
Current assets
Cash and cash equivalents	0		0		0
Current assets held for sale					0
Other current assets	29,265,000		29,265,000		39,078,000
Intercompany receivables	0		0		0
Total current assets	29,265,000		29,265,000		39,078,000
Goodwill	143,012,000		143,012,000		143,012,000
Noncurrent assets held for sale					0
Deferred charges and other assets	30,099,000		30,099,000		27,779,000
Total assets	2,461,733,000		2,461,733,000		2,283,864,000
CAPITALIZATION AND LIABILITIES
Shareholders' equity	812,845,000		812,845,000		715,672,000
Long-term debt	0		0		0
Total capitalization	812,845,000		812,845,000		715,672,000
Current liabilities
Current maturities of long-term debt					0
Short-term debt	0		0		0
Current liabilities held for sale					0
Other current liabilities	38,420,000		38,420,000		39,911,000
Intercompany payables	983,866,000		983,866,000		957,526,000
Total current liabilities	1,022,286,000		1,022,286,000		997,437,000
Deferred income taxes	602,036,000		602,036,000		543,390,000
Regulatory cost of removal obligation	24,529,000		24,529,000		27,119,000
Pension and postretirement liabilities	0		0		0
Noncurrent liabilities held for sale					0
Deferred credits and other liabilities	37,000		37,000		246,000
Total shareholders' equity and liabilities	2,461,733,000		2,461,733,000		2,283,864,000
Pipeline and Storage Segment [Member] | Reportable Subsegments [Member]
Segment Reporting Information Profit Loss [Abstract]
Revenues	32,763,000	31,659,000	84,634,000	85,584,000
Pipeline and Storage Segment [Member] | Intersubsegment Eliminations [Member]
Segment Reporting Information Profit Loss [Abstract]
Revenues	84,520,000	82,196,000	254,573,000	228,840,000
Natural Gas Marketing Segment [Member]
Segment Reporting Information Profit Loss [Abstract]
Total operating revenues	0	0	0	0
Purchased gas cost	0	0	0	0
Operation and maintenance expense	0	0	0	0
Depreciation and amortization expense	0	0	0	0
Taxes, other than income, expense	0	0	0	0
Operating income	0	0	0	0
Miscellaneous income (expense)	0	0	0	0
Interest charges	0	0	0	0
Income (loss) from continuing operations before income taxes	0	0	0	0
Income tax expense (benefit)	0	0	0	0
Income from continuing operations		0	0	0
Gain (Loss) on Disposal of Discontinued Operation, Net of Tax			2,716,000
Net income (loss)	0	5,050,000	13,710,000	5,172,000
Capital expenditures	0	106,000	0	179,000
Income from discontinued operations, net of tax, before gain on disposal		5,050,000	10,994,000	5,172,000
ASSETS
Net property, plant and equipment	0		0		0
Investment in subsidiaries	0		0		0
Current assets
Cash and cash equivalents	0		0		25,417,000
Current assets held for sale					162,508,000
Other current assets	0		0		5,000
Intercompany receivables	0		0		0
Total current assets	0		0		187,930,000
Goodwill	0		0		0
Noncurrent assets held for sale					28,785,000
Deferred charges and other assets	0		0		0
Total assets	0		0		216,715,000
CAPITALIZATION AND LIABILITIES
Shareholders' equity	0		0		66,597,000
Long-term debt	0		0		0
Total capitalization	0		0		66,597,000
Current liabilities
Current maturities of long-term debt					0
Short-term debt	0		0		35,000,000
Current liabilities held for sale					81,908,000
Other current liabilities	0		0		3,263,000
Intercompany payables	0		0		14,139,000
Total current liabilities	0		0		134,310,000
Deferred income taxes	0		0		4,318,000
Regulatory cost of removal obligation	0		0		0
Pension and postretirement liabilities	0		0		0
Noncurrent liabilities held for sale					316,000
Deferred credits and other liabilities	0		0		11,174,000
Total shareholders' equity and liabilities	0		0		216,715,000
Natural Gas Marketing Segment [Member] | Reportable Subsegments [Member]
Segment Reporting Information Profit Loss [Abstract]
Revenues	0	0	0	0
Natural Gas Marketing Segment [Member] | Intersubsegment Eliminations [Member]
Segment Reporting Information Profit Loss [Abstract]
Revenues	0	0	0	0
Intersegment Elimination [Member]
Segment Reporting Information Profit Loss [Abstract]
Total operating revenues	(84,842,000)	(82,548,000)	(255,609,000)	(229,894,000)
Purchased gas cost	(84,842,000)	(82,548,000)	(255,565,000)	(229,894,000)
Operation and maintenance expense	0	0	(44,000)	0
Depreciation and amortization expense	0	0	0	0
Taxes, other than income, expense	0	0	0	0
Operating income	0	0	0	0
Miscellaneous income (expense)	0	0	0	0
Interest charges	0	0	0	0
Income (loss) from continuing operations before income taxes	0	0	0	0
Income tax expense (benefit)	0	0	0	0
Income from continuing operations		0	0	0
Gain (Loss) on Disposal of Discontinued Operation, Net of Tax			0
Net income (loss)	0	0	0	0
Capital expenditures	0	0	0	0
Income from discontinued operations, net of tax, before gain on disposal		0	0	0
ASSETS
Net property, plant and equipment	0		0		0
Investment in subsidiaries	(812,845,000)		(812,845,000)		(782,269,000)
Current assets
Cash and cash equivalents	0		0		0
Current assets held for sale					(11,391,000)
Other current assets	(2,360,000)		(2,360,000)		(46,011,000)
Intercompany receivables	(983,866,000)		(983,866,000)		(971,665,000)
Total current assets	(986,226,000)		(986,226,000)		(1,029,067,000)
Goodwill	0		0		0
Noncurrent assets held for sale					(169,000)
Deferred charges and other assets	0		0		0
Total assets	(1,799,071,000)		(1,799,071,000)		(1,811,505,000)
CAPITALIZATION AND LIABILITIES
Shareholders' equity	(812,845,000)		(812,845,000)		(782,269,000)
Long-term debt	0		0		0
Total capitalization	(812,845,000)		(812,845,000)		(782,269,000)
Current liabilities
Current maturities of long-term debt					0
Short-term debt	0		0		(35,000,000)
Current liabilities held for sale					(9,008,000)
Other current liabilities	(2,360,000)		(2,360,000)		(13,394,000)
Intercompany payables	(983,866,000)		(983,866,000)		(971,665,000)
Total current liabilities	(986,226,000)		(986,226,000)		(1,029,067,000)
Deferred income taxes	0		0		0
Regulatory cost of removal obligation	0		0		0
Pension and postretirement liabilities	0		0		0
Noncurrent liabilities held for sale					0
Deferred credits and other liabilities	0		0		(169,000)
Total shareholders' equity and liabilities	(1,799,071,000)		(1,799,071,000)		$ (1,811,505,000)
Intersegment Elimination [Member] | Reportable Subsegments [Member]
Segment Reporting Information Profit Loss [Abstract]
Revenues	0	0	0	0
Intersegment Elimination [Member] | Intersubsegment Eliminations [Member]
Segment Reporting Information Profit Loss [Abstract]
Revenues	$ (84,842,000)	$ (82,548,000)	$ (255,609,000)	$ (229,894,000)