CONDENSED CONSOLIDATED STATEMENTS OF INCOME (UNAUDITED) - USD ($) shares in Thousands, $ in Thousands	3 Months Ended		6 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Mar. 31, 2018	Mar. 31, 2017
Operating revenues [Abstract]
Operating revenues	$ 1,219,409	$ 988,186	$ 2,108,601	$ 1,768,354
Purchase Gas Cost [Abstract]
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
Gain on sale of discontinued operations, net of tax	0	2,716	0	2,716
Income from discontinued operations, net of tax			0	10,994
Net income (loss)	$ 178,992	$ 164,728	$ 493,124	$ 289,760
Income (Loss) from Continuing Operations, Per Basic and Diluted Share	$ 1.60	$ 1.52	$ 4.47	$ 2.61
Income (Loss) from Discontinued Operation, Net of Tax, Per Basic and Diluted Share	0.00	0.03	0.00	0.13
Basic and diluted net income per share	1.60	1.55	4.47	2.74
Cash dividends per share	$ 0.485	$ 0.450	$ 0.97	$ 0.90
Weighted average shares outstanding:
Weighted Average Number of Shares Outstanding, Basic and Diluted	111,706	105,935	110,135	105,610
Distribution Segment [Member]
Operating revenues [Abstract]
Operating revenues	$ 1,199,291	$ 962,541	$ 2,060,083	$ 1,717,197
Purchase Gas Cost [Abstract]
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
Gain on sale of discontinued operations, net of tax		0		0
Income from discontinued operations, net of tax				0
Net income (loss)	145,243	131,145	394,342	216,509
Pipeline and Storage Segment [Member]
Operating revenues [Abstract]
Operating revenues	120,955	111,972	247,418	221,924
Purchase Gas Cost [Abstract]
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
Gain on sale of discontinued operations, net of tax		0		0
Income from discontinued operations, net of tax				0
Net income (loss)	33,749	30,867	98,782	59,541
Intersegment Elimination [Member]
Operating revenues [Abstract]
Operating revenues	(100,837)	(86,327)	(198,900)	(170,767)
Purchase Gas Cost [Abstract]
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
Gain on sale of discontinued operations, net of tax		0		0
Income from discontinued operations, net of tax				0
Net income (loss)	$ 0	$ 0	$ 0	$ 0