Condensed Consolidated Statements of Income (unaudited) (USD $) In Thousands, unless otherwise specified	3 Months Ended		6 Months Ended
	Mar. 31, 2014	Mar. 31, 2013	Mar. 31, 2014	Mar. 31, 2013
Income Statement [Abstract]
Revenues	$ 513,956	$ 395,901	$ 812,855	$ 669,698
Costs and expenses:
Cost of sales — gas, fuel and purchased power (excluding depreciation shown below)	300,424	212,785	452,289	351,062
Operating and administrative expenses	54,306	52,421	95,451	96,656
Operating and administrative expenses — related parties	3,769	3,012	5,612	4,817
Taxes other than income taxes	4,801	4,684	8,980	8,960
Depreciation	13,815	12,895	27,437	25,648
Amortization	828	817	1,656	1,635
Other (income) loss, net	(1,941)	57	(2,367)	(874)
Total costs and expenses	376,002	286,671	589,058	487,904
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
Income taxes	53,038	41,151	84,798	66,826
Net income	$ 76,110	$ 58,259	$ 121,396	$ 95,071