Consolidated Statements of Income - USD ($) $ in Thousands	12 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2013
Income Statement [Abstract]
Revenues	$ 1,041,581	$ 1,086,889	$ 940,712
Costs and expenses:
Cost of sales — gas, fuel and purchased power (excluding depreciation shown below)	510,784	562,942	465,996
Operating and administrative expenses	206,319	195,408	188,266
Operating and administrative expenses — related parties	11,956	10,671	8,366
Taxes other than income taxes	16,134	16,608	16,877
Depreciation	59,841	55,776	52,298
Amortization	3,749	3,443	3,418
Other income, net	(8,869)	(4,359)	(4,828)
Costs and expenses	799,914	840,489	730,393
Operating income	241,667	246,400	210,319
Interest expense	41,128	38,471	39,309
Income before income taxes	200,539	207,929	171,010
Income taxes	79,484	83,823	68,912
Net income	$ 121,055	$ 124,106	$ 102,098