Condensed Consolidated Statements of Income (unaudited) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
Income Statement [Abstract]
Revenues	$ 143,490	$ 152,694	$ 931,369	$ 965,549
Costs and expenses:
Cost of sales — gas, fuel and purchased power (excluding depreciation shown below)	53,691	63,323	475,079	515,612
Operating and administrative expenses	51,393	49,862	156,858	145,313
Operating and administrative expenses — related parties	2,647	2,385	9,567	7,997
Taxes other than income taxes	3,706	3,768	12,613	12,748
Depreciation	14,985	14,048	44,300	41,485
Amortization	928	844	2,682	2,500
Other operating income, net	(4,044)	(1,256)	(8,253)	(3,623)
Total costs and expenses	123,306	132,974	692,846	722,032
Operating income	20,184	19,720	238,523	243,517
Interest expense	9,985	10,433	31,245	28,036
Income before income taxes	10,199	9,287	207,278	215,481
Income taxes	2,892	2,397	81,543	87,195
Net income	$ 7,307	$ 6,890	$ 125,735	$ 128,286