Consolidated Statements of Income - USD ($) $ in Thousands	12 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2014
Income Statement [Abstract]
Revenues	$ 768,484	$ 1,041,581	$ 1,086,889
Costs and expenses:
Cost of sales — gas, fuel and purchased power (excluding depreciation shown below)	289,786	510,784	562,942
Operating and administrative expenses	180,842	206,319	195,408
Operating and administrative expenses — related parties	11,863	11,956	10,671
Taxes other than income taxes	15,789	16,134	16,608
Depreciation	64,260	59,841	55,776
Amortization	3,043	3,749	3,443
Other expense (income), net	2,000	(8,869)	(4,359)
Costs and expenses	567,583	799,914	840,489
Operating income	200,901	241,667	246,400
Interest expense	37,630	41,128	38,471
Income before income taxes	163,271	200,539	207,929
Income taxes	65,898	79,484	83,823
Net income	$ 97,373	$ 121,055	$ 124,106