Segment Information (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Mar. 31, 2018	Mar. 31, 2017	Sep. 30, 2017
Segment Reporting Information Profit Loss [Abstract]
Total operating revenues	$ 1,219,409	$ 988,186	$ 2,108,601	$ 1,768,354
Purchased gas cost	626,960	427,494	993,877	738,799
Operation and maintenance expense	161,073	132,239	290,640	257,177
Depreciation and amortization expense	89,381	77,667	177,755	154,625
Taxes, other than income, expense	73,007	65,614	135,780	122,663
Operating income	268,988	285,172	510,549	495,090
Miscellaneous income (expense)	(253)	833	(2,288)	(161)
Interest charges	27,304	26,944	58,813	57,974
Income (loss) from continuing operations before income taxes	241,431	259,061	449,448	436,955
Income tax expense (benefit)	62,439	97,049	(43,676)	160,905
Income from continuing operations	178,992	162,012	493,124	276,050
Income from discontinued operations, net of tax			0	10,994
Gain on sale of discontinued operations, net of tax	0	2,716	0	2,716
Net income (loss)	178,992	164,728	493,124	289,760
Capital expenditures	310,740	261,423	693,978	559,385
Segment Reporting Information, Balance Sheet [Abstract]
Net property, plant and equipment	9,761,329		9,761,329		$ 9,259,182
Total assets	11,356,322		11,356,322		10,749,596
Reportable Subsegments [Member]
Segment Reporting Information Profit Loss [Abstract]
Revenues	1,219,409	988,186	2,108,601	1,768,354
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	1,199,291	962,541	2,060,083	1,717,197
Purchased gas cost	727,053	513,096	1,190,811	908,442
Operation and maintenance expense	131,991	103,703	235,728	196,417
Depreciation and amortization expense	65,649	61,302	131,083	122,459
Taxes, other than income, expense	64,692	57,636	119,799	108,182
Operating income	209,906	226,804	382,662	381,697
Miscellaneous income (expense)	393	1,029	(1,007)	396
Interest charges	16,898	16,925	38,266	38,043
Income (loss) from continuing operations before income taxes	193,401	210,908	343,389	344,050
Income tax expense (benefit)	48,158	79,763	(50,953)	127,541
Income from continuing operations		131,145		216,509
Income from discontinued operations, net of tax				0
Gain on sale of discontinued operations, net of tax		0		0
Net income (loss)	145,243	131,145	394,342	216,509
Capital expenditures	224,235	208,185	465,484	430,669
Segment Reporting Information, Balance Sheet [Abstract]
Net property, plant and equipment	7,202,673		7,202,673		6,849,517
Total assets	10,723,398		10,723,398		10,050,164
Distribution Segment [Member] | Reportable Subsegments [Member]
Segment Reporting Information Profit Loss [Abstract]
Revenues	1,198,309	962,217	2,058,762	1,716,483
Distribution Segment [Member] | Intersubsegment Eliminations [Member]
Segment Reporting Information Profit Loss [Abstract]
Revenues	982	324	1,321	714
Pipeline and Storage Segment [Member]
Segment Reporting Information Profit Loss [Abstract]
Total operating revenues	120,955	111,972	247,418	221,924
Purchased gas cost	433	725	1,345	1,080
Operation and maintenance expense	29,393	28,536	55,533	60,804
Depreciation and amortization expense	23,732	16,365	46,672	32,166
Taxes, other than income, expense	8,315	7,978	15,981	14,481
Operating income	59,082	58,368	127,887	113,393
Miscellaneous income (expense)	(646)	(196)	(1,281)	(557)
Interest charges	10,406	10,019	20,547	19,931
Income (loss) from continuing operations before income taxes	48,030	48,153	106,059	92,905
Income tax expense (benefit)	14,281	17,286	7,277	33,364
Income from continuing operations		30,867		59,541
Income from discontinued operations, net of tax				0
Gain on sale of discontinued operations, net of tax		0		0
Net income (loss)	33,749	30,867	98,782	59,541
Capital expenditures	86,505	53,238	228,494	128,716
Segment Reporting Information, Balance Sheet [Abstract]
Net property, plant and equipment	2,558,656		2,558,656		2,409,665
Total assets	2,779,330		2,779,330		2,621,601
Pipeline and Storage Segment [Member] | Reportable Subsegments [Member]
Segment Reporting Information Profit Loss [Abstract]
Revenues	21,100	25,969	49,839	51,871
Pipeline and Storage Segment [Member] | Intersubsegment Eliminations [Member]
Segment Reporting Information Profit Loss [Abstract]
Revenues	99,855	86,003	197,579	170,053
Natural Gas Marketing Segment [Member]
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, 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		0
Income from discontinued operations, net of tax				10,994
Gain on sale of discontinued operations, net of tax		2,716		2,716
Net income (loss)		2,716		13,710
Capital expenditures		0		0
Natural Gas Marketing Segment [Member] | Reportable Subsegments [Member]
Segment Reporting Information Profit Loss [Abstract]
Revenues		0		0
Natural Gas Marketing Segment [Member] | Intersubsegment Eliminations [Member]
Segment Reporting Information Profit Loss [Abstract]
Revenues		0		0
Intersegment Elimination [Member]
Segment Reporting Information Profit Loss [Abstract]
Total operating revenues	(100,837)	(86,327)	(198,900)	(170,767)
Purchased gas cost	(100,526)	(86,327)	(198,279)	(170,723)
Operation and maintenance expense	(311)	0	(621)	(44)
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
Income from discontinued operations, net of tax				0
Gain on sale of discontinued operations, net of tax		0		0
Net income (loss)	0	0	0	0
Capital expenditures	0	0	0	0
Segment Reporting Information, Balance Sheet [Abstract]
Net property, plant and equipment	0		0		0
Total assets	(2,146,406)		(2,146,406)		$ (1,922,169)
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	$ (100,837)	$ (86,327)	$ (198,900)	$ (170,767)