Condensed Consolidated Statements of Income (unaudited) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015
Income Statement [Abstract]
Revenues	$ 140,283	$ 143,490	$ 660,312	$ 931,369
Costs and expenses:
Cost of sales — gas, fuel and purchased power (excluding depreciation shown below)	44,415	53,691	257,288	475,079
Operating and administrative expenses	43,254	51,393	136,406	156,858
Operating and administrative expenses — related parties	2,811	2,647	8,789	9,567
Taxes other than income taxes	3,970	3,706	12,187	12,613
Depreciation	15,877	14,985	47,850	44,300
Amortization	673	928	2,431	2,682
Other operating (income) loss, net	(532)	(4,044)	2,769	(8,253)
Total costs and expenses	110,468	123,306	467,720	692,846
Operating income	29,815	20,184	192,592	238,523
Interest expense	9,158	9,985	27,922	31,245
Income before income taxes	20,657	10,199	164,670	207,278
Income taxes	8,054	2,892	65,422	81,543
Net income	$ 12,603	$ 7,307	$ 99,248	$ 125,735