Condensed Consolidated Statements of Income (unaudited) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Income Statement [Abstract]
Revenues	$ 146,692	$ 140,283	$ 768,045	$ 660,312
Costs and expenses:
Cost of sales — gas, fuel and purchased power (excluding depreciation shown below)	51,979	44,415	325,991	257,288
Operating and administrative expenses	49,031	43,254	148,132	136,406
Operating and administrative expenses — related parties	2,961	2,811	10,059	8,789
Taxes other than income taxes	3,706	3,970	12,342	12,187
Depreciation	17,364	15,877	51,351	47,850
Amortization	548	673	1,651	2,431
Other operating (income) expense, net	(6,568)	(532)	(7,796)	2,769
Total costs and expenses	119,021	110,468	541,730	467,720
Operating income	27,671	29,815	226,315	192,592
Interest expense	10,128	9,158	30,478	27,922
Income before income taxes	17,543	20,657	195,837	164,670
Income taxes	6,846	8,054	75,750	65,422
Net income	$ 10,697	$ 12,603	$ 120,087	$ 99,248