Segment Information (Details) - USD ($) $ in Thousands	3 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Sep. 30, 2017
Segment Reporting Information Profit Loss [Abstract]
Total operating revenues	$ 889,192	$ 780,168
Purchased gas cost	366,917	311,305
Operation and maintenance expense	129,567	124,938
Depreciation and amortization expense	88,374	76,958
Taxes, other than income, expense	62,773	57,049
Operating income	241,561	209,918
Miscellaneous income (expense)	(2,035)	(994)
Interest charges	31,509	31,030
Income (loss) from continuing operations before income taxes	208,017	177,894
Income tax expense (benefit)	(106,115)	63,856
Income from continuing operations	314,132	114,038
Income from discontinued operations, net of tax	0	10,994
Net income (loss)	314,132	125,032
Capital expenditures	383,238	297,962
Segment Reporting Information, Balance Sheet [Abstract]
Net property, plant and equipment	9,518,792		$ 9,259,182
Total assets	11,264,720		10,749,596
Reportable Subsegments [Member]
Segment Reporting Information Profit Loss [Abstract]
Revenues	889,192	780,168
Intersubsegment Eliminations [Member]
Segment Reporting Information Profit Loss [Abstract]
Revenues	0	0
Distribution Segment [Member]
Segment Reporting Information Profit Loss [Abstract]
Total operating revenues	860,792	754,656
Purchased gas cost	463,758	395,346
Operation and maintenance expense	103,737	92,714
Depreciation and amortization expense	65,434	61,157
Taxes, other than income, expense	55,107	50,546
Operating income	172,756	154,893
Miscellaneous income (expense)	(1,400)	(633)
Interest charges	21,368	21,118
Income (loss) from continuing operations before income taxes	149,988	133,142
Income tax expense (benefit)	(99,111)	47,778
Income from continuing operations		85,364
Income from discontinued operations, net of tax		0
Net income (loss)	249,099	85,364
Capital expenditures	241,249	222,484
Segment Reporting Information, Balance Sheet [Abstract]
Net property, plant and equipment	7,010,709		6,849,517
Total assets	10,633,234		10,050,164
Distribution Segment [Member] | Reportable Subsegments [Member]
Segment Reporting Information Profit Loss [Abstract]
Revenues	860,453	754,266
Distribution Segment [Member] | Intersubsegment Eliminations [Member]
Segment Reporting Information Profit Loss [Abstract]
Revenues	339	390
Pipeline and Storage Segment [Member]
Segment Reporting Information Profit Loss [Abstract]
Total operating revenues	126,463	109,952
Purchased gas cost	912	355
Operation and maintenance expense	26,140	32,268
Depreciation and amortization expense	22,940	15,801
Taxes, other than income, expense	7,666	6,503
Operating income	68,805	55,025
Miscellaneous income (expense)	(635)	(361)
Interest charges	10,141	9,912
Income (loss) from continuing operations before income taxes	58,029	44,752
Income tax expense (benefit)	(7,004)	16,078
Income from continuing operations		28,674
Income from discontinued operations, net of tax		0
Net income (loss)	65,033	28,674
Capital expenditures	141,989	75,478
Segment Reporting Information, Balance Sheet [Abstract]
Net property, plant and equipment	2,508,083		2,409,665
Total assets	2,729,455		2,621,601
Pipeline and Storage Segment [Member] | Reportable Subsegments [Member]
Segment Reporting Information Profit Loss [Abstract]
Revenues	28,739	25,902
Pipeline and Storage Segment [Member] | Intersubsegment Eliminations [Member]
Segment Reporting Information Profit Loss [Abstract]
Revenues	97,724	84,050
Natural Gas Marketing Segment [Member]
Segment Reporting Information Profit Loss [Abstract]
Total operating revenues		0
Purchased gas cost		0
Operation and maintenance expense		0
Depreciation and amortization expense		0
Taxes, other than income, expense		0
Operating income		0
Miscellaneous income (expense)		0
Interest charges		0
Income (loss) from continuing operations before income taxes		0
Income tax expense (benefit)		0
Income from continuing operations		0
Income from discontinued operations, net of tax		10,994
Net income (loss)		10,994
Capital expenditures		0
Natural Gas Marketing Segment [Member] | Reportable Subsegments [Member]
Segment Reporting Information Profit Loss [Abstract]
Revenues		0
Natural Gas Marketing Segment [Member] | Intersubsegment Eliminations [Member]
Segment Reporting Information Profit Loss [Abstract]
Revenues		0
Intersegment Elimination [Member]
Segment Reporting Information Profit Loss [Abstract]
Total operating revenues	(98,063)	(84,440)
Purchased gas cost	(97,753)	(84,396)
Operation and maintenance expense	(310)	(44)
Depreciation and amortization expense	0	0
Taxes, other than income, expense	0	0
Operating income	0	0
Miscellaneous income (expense)	0	0
Interest charges	0	0
Income (loss) from continuing operations before income taxes	0	0
Income tax expense (benefit)	0	0
Income from continuing operations		0
Income from discontinued operations, net of tax		0
Net income (loss)	0	0
Capital expenditures	0	0
Segment Reporting Information, Balance Sheet [Abstract]
Net property, plant and equipment	0		0
Total assets	(2,097,969)		$ (1,922,169)
Intersegment Elimination [Member] | Reportable Subsegments [Member]
Segment Reporting Information Profit Loss [Abstract]
Revenues	0	0
Intersegment Elimination [Member] | Intersubsegment Eliminations [Member]
Segment Reporting Information Profit Loss [Abstract]
Revenues	$ (98,063)	$ (84,440)