Condensed Consolidated Statements of Income (unaudited) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Income Statement [Abstract]
Revenues	$ 159,934	$ 146,692	$ 966,300	$ 768,045
Costs and expenses:
Cost of sales — gas and purchased power (excluding depreciation shown below)	72,537	51,979	481,613	325,991
Operating and administrative expenses	58,225	52,737	177,263	160,474
Operating and administrative expenses — related parties	4,325	2,961	10,780	10,059
Depreciation and amortization	21,414	17,912	62,926	53,002
Other operating income, net	(481)	(6,568)	(1,618)	(7,796)
Total costs and expenses	156,020	119,021	730,964	541,730
Operating income	3,914	27,671	235,336	226,315
Interest expense	10,003	10,128	32,033	30,478
(Loss) income before income taxes	(6,089)	17,543	203,303	195,837
Income tax (benefit) expense	(3,066)	6,846	48,839	75,750
Net (loss) income	$ (3,023)	$ 10,697	$ 154,464	$ 120,087