Consolidated Statements of Income (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2012
Income Statement [Abstract]
Revenues	$ 1,086,889	$ 940,712	$ 884,333
Costs and expenses:
Cost of sales — gas, fuel and purchased power (excluding depreciation shown below)	562,942	465,996	459,079
Operating and administrative expenses	195,408	188,266	165,479
Operating and administrative expenses — related parties	10,671	8,366	9,326
Taxes other than income taxes	16,608	16,877	17,263
Depreciation	55,776	52,298	49,702
Amortization	3,443	3,418	3,090
Other income, net	(4,359)	(4,828)	(4,989)
Costs and expenses	840,489	730,393	698,950
Operating income	246,400	210,319	185,383
Interest expense	38,471	39,309	42,412
Income before income taxes	207,929	171,010	142,971
Income taxes	83,823	68,912	55,073
Net income	$ 124,106	$ 102,098	$ 87,898