CONDENSED CONSOLIDATED STATEMENTS OF INCOME (UNAUDITED) - USD ($) shares in Thousands, $ in Thousands	3 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Operating revenues [Abstract]
Operating revenues	$ 889,192	$ 780,168
Purchase Gas Cost [Abstract]
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
Income (Loss) from Continuing Operations, Per Basic and Diluted Share	$ 2.89	$ 1.08
Income (Loss) from Discontinued Operation, Net of Tax, Per Basic and Diluted Share	0.00	0.11
Basic and diluted net income per share	2.89	1.19
Cash dividends per share	$ 0.485	$ 0.450
Weighted average shares outstanding:
Weighted Average Number of Shares Outstanding, Basic and Diluted	108,564	105,284
Distribution Segment [Member]
Operating revenues [Abstract]
Operating revenues	$ 860,792	$ 754,656
Purchase Gas Cost [Abstract]
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
Pipeline and Storage Segment [Member]
Operating revenues [Abstract]
Operating revenues	126,463	109,952
Purchase Gas Cost [Abstract]
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
Intersegment Elimination [Member]
Operating revenues [Abstract]
Operating revenues	(98,063)	(84,440)
Purchase Gas Cost [Abstract]
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