CONSOLIDATED STATEMENTS OF INCOME - USD ($) shares in Thousands, $ in Thousands	3 Months Ended								12 Months Ended
	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2014
Operating revenues
Operating revenues	$ 678,519	$ 632,916	$ 1,132,293	$ 906,221	$ 656,902	$ 686,401	$ 1,540,068	$ 1,258,765	$ 3,349,949	$ 4,142,136	$ 4,940,916
Purchased gas cost
Purchased gas cost									1,605,053	2,462,119	3,358,490
Gross profit	376,011	407,311	517,811	443,763	354,321	381,673	520,738	423,285	1,744,896	1,680,017	1,582,426
Operating expenses
Operation and maintenance									560,766	541,868	505,154
Depreciation and amortization									293,096	274,796	253,987
Taxes, other than income									223,016	231,958	211,936
Total operating expenses									1,076,878	1,048,622	971,077
Operating income	84,633	137,164	250,016	196,205	75,853	117,607	250,210	187,725	668,018	631,395	611,349
Miscellaneous income (expense), net									(1,593)	(4,389)	(5,235)
Interest charges									115,948	116,241	129,295
Income before income taxes									550,477	510,765	476,819
Income tax expense									200,373	195,690	187,002
Net income	$ 34,240	$ 71,193	$ 141,810	$ 102,861	$ 23,515	$ 56,281	$ 137,684	$ 97,595	$ 350,104	$ 315,075	$ 289,817
Basic net income per share	$ 0.33	$ 0.69	$ 1.38	$ 1.00	$ 0.23	$ 0.55	$ 1.35	$ 0.96	$ 3.38	$ 3.09	$ 2.96
Diluted net income per share	$ 0.33	$ 0.69	$ 1.38	$ 1.00	$ 0.23	$ 0.55	$ 1.35	$ 0.96	$ 3.38	$ 3.09	$ 2.96
Weighted average shares outstanding:
Basic (shares)									103,524	101,892	97,606
Diluted (shares)									103,524	101,892	97,608
Regulated Distribution Segment [Member]
Operating revenues
Operating revenues	$ 389,353	$ 414,226	$ 849,685	$ 638,602	$ 369,656	$ 416,794	$ 1,130,613	$ 846,772	$ 2,291,866	$ 2,763,835	$ 3,061,546
Purchased gas cost
Purchased gas cost									1,019,061	1,526,258	1,885,031
Gross profit									1,272,805	1,237,577	1,176,515
Operating expenses
Operation and maintenance									404,115	388,486	387,228
Depreciation and amortization									233,036	223,048	208,376
Taxes, other than income									196,070	205,894	196,343
Total operating expenses									833,221	817,428	791,947
Operating income									439,584	420,149	384,568
Miscellaneous income (expense), net									455	(377)	(381)
Interest charges									79,404	84,132	94,918
Income before income taxes									360,635	335,640	289,269
Income tax expense									128,265	130,827	117,684
Net income									232,370	204,813	171,585
Regulated Pipeline Segment [Member]
Operating revenues
Operating revenues	109,204	109,249	95,703	94,677	97,807	97,008	91,730	83,567	408,833	370,112	318,459
Purchased gas cost
Purchased gas cost									0	0	0
Gross profit									408,833	370,112	318,459
Operating expenses
Operation and maintenance									129,525	118,866	91,466
Depreciation and amortization									55,576	47,236	41,031
Taxes, other than income									24,298	22,743	13,143
Total operating expenses									209,399	188,845	145,640
Operating income									199,434	181,267	172,819
Miscellaneous income (expense), net									(1,683)	(1,243)	(3,181)
Interest charges									36,574	33,151	36,280
Income before income taxes									161,177	146,873	133,358
Income tax expense									59,488	52,211	47,167
Net income									101,689	94,662	86,191
Nonregulated Segment [Member]
Operating revenues
Operating revenues	291,889	214,555	287,395	272,524	292,830	278,769	438,322	462,288	1,066,363	1,472,209	2,067,292
Purchased gas cost
Purchased gas cost									1,002,573	1,399,349	1,979,337
Gross profit									63,790	72,860	87,955
Operating expenses
Operation and maintenance									27,658	35,048	26,963
Depreciation and amortization									4,484	4,512	4,580
Taxes, other than income									2,648	3,321	2,450
Total operating expenses									34,790	42,881	33,993
Operating income									29,000	29,979	53,962
Miscellaneous income (expense), net									1,443	(760)	2,216
Interest charges									1,778	967	1,986
Income before income taxes									28,665	28,252	54,192
Income tax expense									12,620	12,652	22,151
Net income									16,045	15,600	32,041
Intersegment Elimination [Member]
Operating revenues
Operating revenues	$ (111,927)	$ (105,114)	$ (100,490)	$ (99,582)	$ (103,391)	$ (106,170)	$ (120,597)	$ (133,862)	(417,113)	(464,020)	(506,381)
Purchased gas cost
Purchased gas cost									(416,581)	(463,488)	(505,878)
Gross profit									(532)	(532)	(503)
Operating expenses
Operation and maintenance									(532)	(532)	(503)
Depreciation and amortization									0	0	0
Taxes, other than income									0	0	0
Total operating expenses									(532)	(532)	(503)
Operating income									0	0	0
Miscellaneous income (expense), net									(1,808)	(2,009)	(3,889)
Interest charges									(1,808)	(2,009)	(3,889)
Income before income taxes									0	0	0
Income tax expense									0	0	0
Net income									$ 0	$ 0	$ 0