Segment Information - Segment Information (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended		6 Months Ended
	Mar. 31, 2014 Segment	Mar. 31, 2013	Mar. 31, 2014	Mar. 31, 2013	Sep. 30, 2013
Segment Reporting Information
Revenues	$ 513,956	$ 395,901	$ 812,855	$ 669,698
Cost of sales	300,424	212,785	452,289	351,062
Depreciation and amortization	14,643	13,712	29,093	27,283
Operating income	137,954	109,230	223,797	181,794
Interest expense	8,806	9,820	17,603	19,897
Income before income taxes	129,148	99,410	206,194	161,897
Total assets (at period end)	2,340,617	2,348,337	2,340,617	2,348,337	2,210,322
Goodwill (at period end)	182,145	182,145	182,145	182,145	182,145
Capital expenditures	31,648	25,791	65,902	55,519
Number of reportable segments (in segments)	2
Concentration risk, percentage	10.00%
Gas Utility
Segment Reporting Information
Revenues	480,163	368,635	751,725	616,889
Cost of sales	278,748	196,742	414,235	320,316
Depreciation and amortization	13,575	12,731	26,959	25,336
Operating income	134,560	105,749	216,613	175,538
Interest expense	8,362	9,327	16,748	18,899
Income before income taxes	126,198	96,422	199,865	156,639
Total assets (at period end)	2,195,437	2,184,816	2,195,437	2,184,816
Goodwill (at period end)	182,145	182,145	182,145	182,145
Capital expenditures	30,039	24,430	62,851	52,949
Electric Utility
Segment Reporting Information
Revenues	33,552	26,951	60,513	52,069
Cost of sales	21,676	16,043	38,054	30,746
Depreciation and amortization	1,068	981	2,134	1,947
Operating income	3,492	3,389	7,181	6,013
Interest expense	444	493	855	998
Income before income taxes	3,048	2,896	6,326	5,015
Total assets (at period end)	145,180	163,521	145,180	163,521
Goodwill (at period end)	0	0	0	0
Capital expenditures	1,609	1,361	3,051	2,570
Counties of operation, number	2
Other
Segment Reporting Information
Revenues	241	315	617	740
Cost of sales	0	0	0	0
Depreciation and amortization	0	0	0	0
Operating income	(98)	92	3	243
Interest expense	0	0	0	0
Income before income taxes	(98)	92	3	243
Total assets (at period end)	0	0	0	0
Goodwill (at period end)	0	0	0	0
Capital expenditures	$ 0	$ 0	$ 0	$ 0
Customer Concentration Risk | Revenue
Segment Reporting Information
Number of customers exceeding threshold	0