Consolidated Statements of Income - USD ($) $ in Thousands	12 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2015
Income Statement [Abstract]
Revenues	$ 887,588	$ 768,484	$ 1,041,581
Costs and expenses:
Cost of sales — gas, fuel and purchased power (excluding depreciation shown below)	367,279	289,786	510,784
Operating and administrative expenses	199,997	180,842	206,319
Operating and administrative expenses — related parties	12,354	11,863	11,956
Taxes other than income taxes	15,648	15,789	16,134
Depreciation	69,778	64,260	59,841
Amortization	2,554	3,043	3,749
Other operating (income) expense, net	(8,329)	2,000	(8,869)
Costs and expenses	659,281	567,583	799,914
Operating income	228,307	200,901	241,667
Interest expense	40,212	37,630	41,128
Income before income taxes	188,095	163,271	200,539
Income taxes	72,054	65,898	79,484
Net income	$ 116,041	$ 97,373	$ 121,055