CONDENSED CONSOLIDATED STATEMENTS OF INCOME (UNAUDITED) - USD ($) shares in Thousands, $ in Thousands	3 Months Ended		9 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Operating revenues [Abstract]
Operating revenues	$ 526,501	$ 456,212	$ 2,294,855	$ 2,021,005
Purchase Gas Cost [Abstract]
Purchased gas cost	114,176	64,583	852,975	682,265
Operation and maintenance expense	128,690	131,388	385,867	379,073
Depreciation and amortization expense	80,023	72,880	234,648	214,927
Taxes, other than income, expense	62,948	58,965	185,611	171,959
Operating income	140,664	128,396	635,754	572,781
Miscellaneous income (expense)	(289)	1,118	(450)	(90)
Interest charges	28,498	27,679	86,472	84,775
Income (loss) from continuing operations before income taxes	111,877	101,835	548,832	487,916
Income tax expense (benefit)	41,069	35,692	201,974	177,224
Income from continuing operations	70,808	66,143	346,858	310,692
Income from discontinued operations, net of tax, before gain on disposal	0	5,050	10,994	5,172
Gain (Loss) on Disposal of Discontinued Operation, Net of Tax			2,716	0
Income from discontinued operations, net of tax	0	5,050	13,710	5,172
Net income (loss)	$ 70,808	$ 71,193	$ 360,568	$ 315,864
Income (Loss) from Continuing Operations, Per Basic and Diluted Share	$ 0.67	$ 0.64	$ 3.27	$ 3.01
Income (Loss) from Discontinued Operation, Net of Tax, Per Basic and Diluted Share	0.00	0.05	0.13	0.05
Basic and diluted net income per share	0.67	0.69	3.40	3.06
Cash dividends per share	$ 0.45	$ 0.42	$ 1.35	$ 1.26
Weighted average shares outstanding:
Weighted Average Number of Shares Outstanding, Basic and Diluted	106,364	103,750	105,862	103,137
Distribution Segment [Member]
Operating revenues [Abstract]
Operating revenues	$ 494,060	$ 424,905	$ 2,211,257	$ 1,936,475
Purchase Gas Cost [Abstract]
Purchased gas cost	197,767	147,569	1,106,209	912,231
Operation and maintenance expense	99,631	101,819	296,048	294,154
Depreciation and amortization expense	62,760	59,193	185,219	174,748
Taxes, other than income, expense	56,850	52,662	165,032	153,198
Operating income	77,052	63,662	458,749	402,144
Miscellaneous income (expense)	(62)	1,243	334	804
Interest charges	18,394	18,677	56,437	57,481
Income (loss) from continuing operations before income taxes	58,596	46,228	402,646	345,467
Income tax expense (benefit)	22,082	15,867	149,623	126,090
Income from continuing operations		30,361	253,023	219,377
Income from discontinued operations, net of tax, before gain on disposal		0	0	0
Gain (Loss) on Disposal of Discontinued Operation, Net of Tax			0
Net income (loss)	36,514	30,361	253,023	219,377
Pipeline and Storage Segment [Member]
Operating revenues [Abstract]
Operating revenues	117,283	113,855	339,207	314,424
Purchase Gas Cost [Abstract]
Purchased gas cost	1,251	(438)	2,331	(72)
Operation and maintenance expense	29,059	29,569	89,863	84,919
Depreciation and amortization expense	17,263	13,687	49,429	40,179
Taxes, other than income, expense	6,098	6,303	20,579	18,761
Operating income	63,612	64,734	177,005	170,637
Miscellaneous income (expense)	(227)	(125)	(784)	(894)
Interest charges	10,104	9,002	30,035	27,294
Income (loss) from continuing operations before income taxes	53,281	55,607	146,186	142,449
Income tax expense (benefit)	18,987	19,825	52,351	51,134
Income from continuing operations		35,782	93,835	91,315
Income from discontinued operations, net of tax, before gain on disposal		0	0	0
Gain (Loss) on Disposal of Discontinued Operation, Net of Tax			0
Net income (loss)	34,294	35,782	93,835	91,315
Intersegment Elimination [Member]
Operating revenues [Abstract]
Operating revenues	(84,842)	(82,548)	(255,609)	(229,894)
Purchase Gas Cost [Abstract]
Purchased gas cost	(84,842)	(82,548)	(255,565)	(229,894)
Operation and maintenance expense	0	0	(44)	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
Income from discontinued operations, net of tax, before gain on disposal		0	0	0
Gain (Loss) on Disposal of Discontinued Operation, Net of Tax			0
Net income (loss)	$ 0	$ 0	$ 0	$ 0