CONDENSED CONSOLIDATED STATEMENTS OF INCOME (UNAUDITED) - USD ($) shares in Thousands, $ in Thousands	3 Months Ended		6 Months Ended
	Mar. 31, 2016	Mar. 31, 2015	Mar. 31, 2016	Mar. 31, 2015
Operating revenues
Operating revenues	$ 1,132,293	$ 1,540,068	$ 2,038,514	$ 2,798,833
Purchased gas cost
Purchased gas cost	614,482	1,019,330	1,076,940	1,854,810
Gross profit	517,811	520,738	961,574	944,023
Operating expenses
Operation and maintenance	133,666	133,460	258,514	252,042
Depreciation and amortization	71,972	68,022	143,211	135,615
Taxes, other than income	62,157	69,046	113,628	118,431
Total operating expenses	267,795	270,528	515,353	506,088
Operating income	250,016	250,210	446,221	437,935
Miscellaneous income (expense)	(685)	(1,561)	(1,894)	(3,268)
Interest charges	27,560	27,447	58,043	57,211
Income (loss) before income taxes	221,771	221,202	386,284	377,456
Income tax expense (benefit)	79,961	83,518	141,613	142,177
Net income (loss)	$ 141,810	$ 137,684	$ 244,671	$ 235,279
Basic and diluted net income per share	$ 1.38	$ 1.35	$ 2.38	$ 2.31
Cash dividends per share	$ 0.42	$ 0.39	$ 0.84	$ 0.78
Weighted average shares outstanding:
Weighted Average Number of Shares Outstanding, Basic and Diluted	102,946	101,746	102,837	101,667
Regulated Distribution Segment [Member]
Operating revenues
Operating revenues	$ 849,685	$ 1,130,613	$ 1,488,287	$ 1,977,385
Purchased gas cost
Purchased gas cost	440,543	724,378	745,684	1,247,338
Gross profit	409,142	406,235	742,603	730,047
Operating expenses
Operation and maintenance	99,180	103,425	190,529	190,410
Depreciation and amortization	57,663	55,153	114,997	110,239
Taxes, other than income	54,686	62,939	99,947	106,583
Total operating expenses	211,529	221,517	405,473	407,232
Operating income	197,613	184,718	337,130	322,815
Miscellaneous income (expense)	(150)	(937)	(902)	(2,266)
Interest charges	18,717	19,313	39,422	40,953
Income (loss) before income taxes	178,746	164,468	296,806	279,596
Income tax expense (benefit)	64,434	62,615	109,239	106,356
Net income (loss)	114,312	101,853	187,567	173,240
Regulated Pipeline Segment [Member]
Operating revenues
Operating revenues	95,703	91,730	190,380	175,297
Purchased gas cost
Purchased gas cost	0	0	0	0
Gross profit	95,703	91,730	190,380	175,297
Operating expenses
Operation and maintenance	27,131	22,842	54,219	47,457
Depreciation and amortization	13,179	11,747	25,949	23,129
Taxes, other than income	6,738	5,238	12,309	10,103
Total operating expenses	47,048	39,827	92,477	80,689
Operating income	48,655	51,903	97,903	94,608
Miscellaneous income (expense)	(376)	(379)	(805)	(631)
Interest charges	9,145	8,391	18,292	16,715
Income (loss) before income taxes	39,134	43,133	78,806	77,262
Income tax expense (benefit)	13,949	15,451	28,035	27,545
Net income (loss)	25,185	27,682	50,771	49,717
Nonregulated Segment [Member]
Operating revenues
Operating revenues	287,395	438,322	559,919	900,610
Purchased gas cost
Purchased gas cost	274,296	415,416	531,062	861,665
Gross profit	13,099	22,906	28,857	38,945
Operating expenses
Operation and maintenance	7,488	7,326	14,032	14,441
Depreciation and amortization	1,130	1,122	2,265	2,247
Taxes, other than income	733	869	1,372	1,745
Total operating expenses	9,351	9,317	17,669	18,433
Operating income	3,748	13,589	11,188	20,512
Miscellaneous income (expense)	292	252	671	552
Interest charges	149	240	1,187	466
Income (loss) before income taxes	3,891	13,601	10,672	20,598
Income tax expense (benefit)	1,578	5,452	4,339	8,276
Net income (loss)	2,313	8,149	6,333	12,322
Intersegment Elimination [Member]
Operating revenues
Operating revenues	(100,490)	(120,597)	(200,072)	(254,459)
Purchased gas cost
Purchased gas cost	(100,357)	(120,464)	(199,806)	(254,193)
Gross profit	(133)	(133)	(266)	(266)
Operating expenses
Operation and maintenance	(133)	(133)	(266)	(266)
Depreciation and amortization	0	0	0	0
Taxes, other than income	0	0	0	0
Total operating expenses	(133)	(133)	(266)	(266)
Operating income	0	0	0	0
Miscellaneous income (expense)	(451)	(497)	(858)	(923)
Interest charges	(451)	(497)	(858)	(923)
Income (loss) before income taxes	0	0	0	0
Income tax expense (benefit)	0	0	0	0
Net income (loss)	$ 0	$ 0	$ 0	$ 0