Segment Information (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Sep. 30, 2017
Segment Reporting Information Profit Loss [Abstract]
Total operating revenues	$ 562,245	$ 526,501	$ 2,670,846	$ 2,294,855
Purchased gas cost	130,886	114,176	1,124,763	852,975
Operation and maintenance expense	145,075	128,690	435,715	385,867
Depreciation and amortization expense	90,671	80,023	268,426	234,648
Taxes, other than income	72,620	62,948	208,400	185,611
Operating income	122,993	140,664	633,542	635,754
Miscellaneous expense	(2,003)	(289)	(4,291)	(450)
Interest charges	23,349	28,498	82,162	86,472
Income before income taxes	97,641	111,877	547,089	548,832
Income tax (benefit) expense	26,448	41,069	(17,228)	201,974
Income from continuing operations	71,193	70,808	564,317	346,858
Income from discontinued operations, net of tax			0	10,994
Gain on sale of discontinued operations, net of tax			0	2,716
Net income	71,193	70,808	564,317	360,568
Capital expenditures	394,494	252,763	1,088,472	812,148
Segment Reporting Information, Balance Sheet [Abstract]
Property, plant and equipment, net	10,071,860		10,071,860		$ 9,259,182
Total assets	11,507,624		11,507,624		10,749,596
Operating Segments | Distribution
Segment Reporting Information Profit Loss [Abstract]
Total operating revenues	535,488	494,060	2,595,571	2,211,257
Purchased gas cost	230,887	197,767	1,421,698	1,106,209
Operation and maintenance expense	111,895	99,631	347,623	296,048
Depreciation and amortization expense	66,504	62,760	197,587	185,219
Taxes, other than income	64,420	56,850	184,219	165,032
Operating income	61,782	77,052	444,444	458,749
Miscellaneous expense	(1,191)	(62)	(2,198)	334
Interest charges	13,315	18,394	51,581	56,437
Income before income taxes	47,276	58,596	390,665	402,646
Income tax (benefit) expense	11,932	22,082	(39,021)	149,623
Income from continuing operations				253,023
Income from discontinued operations, net of tax				0
Gain on sale of discontinued operations, net of tax				0
Net income	35,344	36,514	429,686	253,023
Capital expenditures	284,209	205,780	749,693	636,449
Segment Reporting Information, Balance Sheet [Abstract]
Property, plant and equipment, net	7,427,486		7,427,486		6,849,517
Total assets	10,840,846		10,840,846		10,050,164
Operating Segments | Pipeline and Storage
Segment Reporting Information Profit Loss [Abstract]
Total operating revenues	127,633	117,283	375,051	339,207
Purchased gas cost	561	1,251	1,906	2,331
Operation and maintenance expense	33,494	29,059	89,027	89,863
Depreciation and amortization expense	24,167	17,263	70,839	49,429
Taxes, other than income	8,200	6,098	24,181	20,579
Operating income	61,211	63,612	189,098	177,005
Miscellaneous expense	(812)	(227)	(2,093)	(784)
Interest charges	10,034	10,104	30,581	30,035
Income before income taxes	50,365	53,281	156,424	146,186
Income tax (benefit) expense	14,516	18,987	21,793	52,351
Income from continuing operations				93,835
Income from discontinued operations, net of tax				0
Gain on sale of discontinued operations, net of tax				0
Net income	35,849	34,294	134,631	93,835
Capital expenditures	110,285	46,983	338,779	175,699
Segment Reporting Information, Balance Sheet [Abstract]
Property, plant and equipment, net	2,644,374		2,644,374		2,409,665
Total assets	2,866,266		2,866,266		2,621,601
Operating Segments | Natural Gas Marketing
Segment Reporting Information Profit Loss [Abstract]
Total operating revenues		0		0
Purchased gas cost		0		0
Operation and maintenance expense		0		0
Depreciation and amortization expense		0		0
Taxes, other than income		0		0
Operating income		0		0
Miscellaneous expense		0		0
Interest charges		0		0
Income before income taxes		0		0
Income tax (benefit) expense		0		0
Income from continuing operations				0
Income from discontinued operations, net of tax				10,994
Gain on sale of discontinued operations, net of tax				2,716
Net income		0		13,710
Capital expenditures		0		0
Eliminations
Segment Reporting Information Profit Loss [Abstract]
Total operating revenues	(100,876)	(84,842)	(299,776)	(255,609)
Purchased gas cost	(100,562)	(84,842)	(298,841)	(255,565)
Operation and maintenance expense	(314)	0	(935)	(44)
Depreciation and amortization expense	0	0	0	0
Taxes, other than income	0	0	0	0
Operating income	0	0	0	0
Miscellaneous expense	0	0	0	0
Interest charges	0	0	0	0
Income before income taxes	0	0	0	0
Income tax (benefit) expense	0	0	0	0
Income from continuing operations				0
Income from discontinued operations, net of tax				0
Gain on sale of discontinued operations, net of tax				0
Net income	0	0	0	0
Capital expenditures	0	0	0	0
Segment Reporting Information, Balance Sheet [Abstract]
Property, plant and equipment, net	0		0		0
Total assets	(2,199,488)		(2,199,488)		$ (1,922,169)
Reportable Subsegments
Segment Reporting Information Profit Loss [Abstract]
Total operating revenues	562,245	526,501	2,670,846	2,294,855
Reportable Subsegments | Distribution
Segment Reporting Information Profit Loss [Abstract]
Total operating revenues	534,816	493,738	2,593,578	2,210,221
Reportable Subsegments | Pipeline and Storage
Segment Reporting Information Profit Loss [Abstract]
Total operating revenues	27,429	32,763	77,268	84,634
Reportable Subsegments | Natural Gas Marketing
Segment Reporting Information Profit Loss [Abstract]
Total operating revenues		0		0
Reportable Subsegments | Eliminations
Segment Reporting Information Profit Loss [Abstract]
Total operating revenues	0	0	0	0
Intersubsegment Eliminations
Segment Reporting Information Profit Loss [Abstract]
Total operating revenues	0	0	0	0
Intersubsegment Eliminations | Distribution
Segment Reporting Information Profit Loss [Abstract]
Total operating revenues	672	322	1,993	1,036
Intersubsegment Eliminations | Pipeline and Storage
Segment Reporting Information Profit Loss [Abstract]
Total operating revenues	100,204	84,520	297,783	254,573
Intersubsegment Eliminations | Natural Gas Marketing
Segment Reporting Information Profit Loss [Abstract]
Total operating revenues		0		0
Intersubsegment Eliminations | Eliminations
Segment Reporting Information Profit Loss [Abstract]
Total operating revenues	$ (100,876)	$ (84,842)	$ (299,776)	$ (255,609)