CONDENSED CONSOLIDATED STATEMENTS OF INCOME (UNAUDITED) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended		6 Months Ended
	Mar. 31, 2013	Mar. 31, 2012	Mar. 31, 2013	Mar. 31, 2012
Operating revenues
Operating revenues	$ 1,308,996	$ 1,225,509	$ 2,343,151	$ 2,309,503
Purchased gas cost
Purchased gas cost	876,245	799,722	1,548,038	1,528,324
Gross profit	432,751	425,787	795,113	781,179
Operating expenses
Operation and maintenance	111,086	109,300	217,613	223,944
Depreciation and amortization	57,180	59,420	116,759	117,786
Taxes, other than income	54,307	54,635	95,641	97,546
Total operating expenses	222,573	223,355	430,013	439,276
Operating income	210,178	202,432	365,100	341,903
Miscellaneous income (expense)	1,712	506	2,410	(1,510)
Interest charges	33,331	36,643	63,853	72,369
Income (loss) before income taxes	178,559	166,295	303,657	268,024
Income tax expense (benefit)	66,219	64,211	113,969	103,556
Income (loss) from continuing operations	112,340	102,084	189,688	164,468
Discontinued Operation Income Loss From Discontinued Operation During Phase Out Period Net Of Tax	4,085	7,027	7,202	13,150
Net income (loss)	116,425	109,111	196,890	177,618
Income Loss From Continuing Operations Per Basic Share	$ 1.24	$ 1.12	$ 2.09	$ 1.81
Income Loss From Discontinued Operations Net Of Tax Per Basic Share	$ 0.04	$ 0.08	$ 0.08	$ 0.14
Basic net income (loss) per share	$ 1.28	$ 1.20	$ 2.17	$ 1.95
Income Loss From Continuing Operations Per Diluted Share	$ 1.23	$ 1.12	$ 2.07	$ 1.80
Income Loss From Discontinued Operations Net Of Tax Per Diluted Share	$ 0.04	$ 0.08	$ 0.08	$ 0.14
Diluted net income (loss) per share	$ 1.27	$ 1.20	$ 2.15	$ 1.94
Cash dividends per share	$ 0.350	$ 0.345	$ 0.700	$ 0.690
Weighted average shares outstanding:
Basic	90,530	90,020	90,445	90,137
Diluted	91,492	90,322	91,406	90,440
Natural Gas Distribution Segment [Member]
Operating revenues
Operating revenues	905,176	871,067	1,571,963	1,547,180
Purchased gas cost
Purchased gas cost	558,170	498,739	945,326	891,257
Gross profit	347,006	372,328	626,637	655,923
Operating expenses
Operation and maintenance	89,344	88,035	173,080	180,031
Depreciation and amortization	47,631	50,903	97,691	100,885
Taxes, other than income	49,592	50,029	86,343	88,221
Total operating expenses	186,567	188,967	357,114	369,137
Operating income	160,439	183,361	269,523	286,786
Miscellaneous income (expense)	2,591	623	2,460	(1,274)
Interest charges	25,664	28,816	49,227	56,955
Income (loss) before income taxes	137,366	155,168	222,756	228,557
Income tax expense (benefit)	51,176	60,693	83,473	89,581
Income (loss) from continuing operations	86,190	94,475	139,283	138,976
Discontinued Operation Income Loss From Discontinued Operation During Phase Out Period Net Of Tax	4,085	7,027	7,202	13,150
Net income (loss)	90,275	101,502	146,485	152,126
Regulated Transmission and Storage Segment [Member]
Operating revenues
Operating revenues	61,848	58,037	122,529	114,796
Purchased gas cost
Purchased gas cost	0	0	0	0
Gross profit	61,848	58,037	122,529	114,796
Operating expenses
Operation and maintenance	15,390	15,847	31,710	32,812
Depreciation and amortization	8,690	7,792	17,080	15,443
Taxes, other than income	4,277	3,915	8,226	7,699
Total operating expenses	28,357	27,554	57,016	55,954
Operating income	33,491	30,483	65,513	58,842
Miscellaneous income (expense)	(99)	(56)	(226)	(336)
Interest charges	7,857	7,614	14,728	14,823
Income (loss) before income taxes	25,535	22,813	50,559	43,683
Income tax expense (benefit)	9,005	8,193	17,924	15,649
Income (loss) from continuing operations	16,530	14,620	32,635	28,034
Discontinued Operation Income Loss From Discontinued Operation During Phase Out Period Net Of Tax	0	0	0	0
Net income (loss)	16,530	14,620	32,635	28,034
Nonregulated Segment [Member]
Operating revenues
Operating revenues	428,948	370,763	828,842	814,939
Purchased gas cost
Purchased gas cost	404,641	374,992	782,076	803,763
Gross profit	24,307	(4,229)	46,766	11,176
Operating expenses
Operation and maintenance	6,763	5,769	13,645	11,820
Depreciation and amortization	859	725	1,988	1,458
Taxes, other than income	438	691	1,072	1,626
Total operating expenses	8,060	7,185	16,705	14,904
Operating income	16,247	(11,414)	30,061	(3,728)
Miscellaneous income (expense)	(91)	567	1,576	603
Interest charges	498	839	1,295	1,091
Income (loss) before income taxes	15,658	(11,686)	30,342	(4,216)
Income tax expense (benefit)	6,038	(4,675)	12,572	(1,674)
Income (loss) from continuing operations	9,620	(7,011)	17,770	(2,542)
Discontinued Operation Income Loss From Discontinued Operation During Phase Out Period Net Of Tax	0	0	0	0
Net income (loss)	9,620	(7,011)	17,770	(2,542)
Intersegment Elimination [Member]
Operating revenues
Operating revenues	(86,976)	(74,358)	(180,183)	(167,412)
Purchased gas cost
Purchased gas cost	(86,566)	(74,009)	(179,364)	(166,696)
Gross profit	(410)	(349)	(819)	(716)
Operating expenses
Operation and maintenance	(411)	(351)	(822)	(719)
Depreciation and amortization	0	0	0	0
Taxes, other than income	0	0	0	0
Total operating expenses	(411)	(351)	(822)	(719)
Operating income	1	2	3	3
Miscellaneous income (expense)	(689)	(628)	(1,400)	(503)
Interest charges	(688)	(626)	(1,397)	(500)
Income (loss) before income taxes	0	0	0	0
Income tax expense (benefit)	0	0	0	0
Income (loss) from continuing operations	0	0	0	0
Discontinued Operation Income Loss From Discontinued Operation During Phase Out Period Net Of Tax	0	0	0	0
Net income (loss)	$ 0	$ 0	$ 0	$ 0