CONDENSED CONSOLIDATED STATEMENTS OF INCOME (UNAUDITED) (USD $) Share data in Thousands, except Per Share data, unless otherwise specified	3 Months Ended		6 Months Ended
	Mar. 31, 2014	Mar. 31, 2013	Mar. 31, 2014	Mar. 31, 2013
Operating revenues
Operating revenues	$ 1,964,322,000	$ 1,308,996,000	$ 3,219,470,000	$ 2,343,151,000
Purchased gas cost
Purchased gas cost	1,468,045,000	876,245,000	2,334,236,000	1,548,038,000
Gross profit	496,277,000	432,751,000	885,234,000	795,113,000
Operating expenses
Operation and maintenance	124,675,000	111,086,000	240,432,000	217,613,000
Depreciation and amortization	61,307,000	57,180,000	121,776,000	116,759,000
Taxes, other than income	60,215,000	54,307,000	102,226,000	95,641,000
Total operating expenses	246,197,000	222,573,000	464,434,000	430,013,000
Operating income	250,080,000	210,178,000	420,800,000	365,100,000
Miscellaneous income (expense)	(1,516,000)	1,712,000	(3,648,000)	2,410,000
Interest charges	31,601,000	33,331,000	63,716,000	63,853,000
Income (loss) before income taxes	216,963,000	178,559,000	353,436,000	303,657,000
Income tax expense (benefit)	83,596,000	66,219,000	133,053,000	113,969,000
Income (loss) from continuing operations	133,367,000	112,340,000	220,383,000	189,688,000
Income from discontinued operations, net of tax	0	4,085,000	0	7,202,000
Net income (loss)	133,367,000	116,425,000	220,383,000	196,890,000
Income (Loss) From Continuing Operations Per Basic Share	$ 1.40	$ 1.24	$ 2.36	$ 2.09
Income (Loss) From Discontinued Operations Net Of Tax Per Basic Share	$ 0.00	$ 0.04	$ 0.00	$ 0.08
Basic net income (loss) per share	$ 1.40	$ 1.28	$ 2.36	$ 2.17
Income (Loss) From Continuing Operations Per Diluted Share	$ 1.38	$ 1.23	$ 2.34	$ 2.07
Income (Loss) From Discontinued Operations Net Of Tax Per Diluted Share	$ 0.00	$ 0.04	$ 0.00	$ 0.08
Diluted net income (loss) per share	$ 1.38	$ 1.27	$ 2.34	$ 2.15
Cash dividends per share	$ 0.37	$ 0.35	$ 0.74	$ 0.7
Weighted average shares outstanding:
Basic weighted average shares outstanding	95,264	90,530	93,049	90,445
Diluted weighted average shares outstanding	96,191	91,492	93,976	91,406
Natural Gas Distribution Segment [Member]
Operating revenues
Operating revenues	1,290,960,000	905,176,000	2,134,825,000	1,571,963,000
Purchased gas cost
Purchased gas cost	905,772,000	558,170,000	1,450,466,000	945,326,000
Gross profit	385,188,000	347,006,000	684,359,000	626,637,000
Operating expenses
Operation and maintenance	106,776,000	89,344,000	196,439,000	173,080,000
Depreciation and amortization	50,020,000	47,631,000	99,571,000	97,691,000
Taxes, other than income	60,606,000	49,592,000	97,690,000	86,343,000
Total operating expenses	217,402,000	186,567,000	393,700,000	357,114,000
Operating income	167,786,000	160,439,000	290,659,000	269,523,000
Miscellaneous income (expense)	97,000	2,591,000	(374,000)	2,460,000
Interest charges	22,828,000	25,664,000	46,153,000	49,227,000
Income (loss) before income taxes	145,055,000	137,366,000	244,132,000	222,756,000
Income tax expense (benefit)	56,312,000	51,176,000	92,632,000	83,473,000
Income (loss) from continuing operations		86,190,000		139,283,000
Income from discontinued operations, net of tax		4,085,000		7,202,000
Net income (loss)	88,743,000	90,275,000	151,500,000	146,485,000
Regulated Transmission and Storage Segment [Member]
Operating revenues
Operating revenues	73,615,000	61,848,000	144,956,000	122,529,000
Purchased gas cost
Purchased gas cost	0	0	0	0
Gross profit	73,615,000	61,848,000	144,956,000	122,529,000
Operating expenses
Operation and maintenance	16,595,000	15,390,000	33,895,000	31,710,000
Depreciation and amortization	10,156,000	8,690,000	19,942,000	17,080,000
Taxes, other than income	(1,232,000)	4,277,000	3,431,000	8,226,000
Total operating expenses	25,519,000	28,357,000	57,268,000	57,016,000
Operating income	48,096,000	33,491,000	87,688,000	65,513,000
Miscellaneous income (expense)	(1,081,000)	(99,000)	(2,262,000)	(226,000)
Interest charges	9,155,000	7,857,000	18,112,000	14,728,000
Income (loss) before income taxes	37,860,000	25,535,000	67,314,000	50,559,000
Income tax expense (benefit)	13,751,000	9,005,000	23,759,000	17,924,000
Income (loss) from continuing operations		16,530,000		32,635,000
Income from discontinued operations, net of tax		0		0
Net income (loss)	24,109,000	16,530,000	43,555,000	32,635,000
Nonregulated Segment [Member]
Operating revenues
Operating revenues	757,683,000	428,948,000	1,205,404,000	828,842,000
Purchased gas cost
Purchased gas cost	720,094,000	404,641,000	1,149,249,000	782,076,000
Gross profit	37,589,000	24,307,000	56,155,000	46,766,000
Operating expenses
Operation and maintenance	1,419,000	6,763,000	10,334,000	13,645,000
Depreciation and amortization	1,131,000	859,000	2,263,000	1,988,000
Taxes, other than income	841,000	438,000	1,105,000	1,072,000
Total operating expenses	3,391,000	8,060,000	13,702,000	16,705,000
Operating income	34,198,000	16,247,000	42,453,000	30,061,000
Miscellaneous income (expense)	443,000	(91,000)	767,000	1,576,000
Interest charges	593,000	498,000	1,230,000	1,295,000
Income (loss) before income taxes	34,048,000	15,658,000	41,990,000	30,342,000
Income tax expense (benefit)	13,533,000	6,038,000	16,662,000	12,572,000
Income (loss) from continuing operations		9,620,000		17,770,000
Income from discontinued operations, net of tax		0		0
Net income (loss)	20,515,000	9,620,000	25,328,000	17,770,000
Intersegment Elimination [Member]
Operating revenues
Operating revenues	(157,936,000)	(86,976,000)	(265,715,000)	(180,183,000)
Purchased gas cost
Purchased gas cost	(157,821,000)	(86,566,000)	(265,479,000)	(179,364,000)
Gross profit	(115,000)	(410,000)	(236,000)	(819,000)
Operating expenses
Operation and maintenance	(115,000)	(411,000)	(236,000)	(822,000)
Depreciation and amortization	0	0	0	0
Taxes, other than income	0	0	0	0
Total operating expenses	(115,000)	(411,000)	(236,000)	(822,000)
Operating income	0	1,000	0	3,000
Miscellaneous income (expense)	(975,000)	(689,000)	(1,779,000)	(1,400,000)
Interest charges	(975,000)	(688,000)	(1,779,000)	(1,397,000)
Income (loss) before income taxes	0	0	0	0
Income tax expense (benefit)	0	0	0	0
Income (loss) from continuing operations		0		0
Income from discontinued operations, net of tax		0		0
Net income (loss)	$ 0	$ 0	$ 0	$ 0