Consolidated Statements of Income (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2011
Income Statement [Abstract]
Revenues	$ 940,712	$ 884,333	$ 1,137,366
Costs and expenses:
Cost of sales — gas, fuel and purchased power (excluding depreciation shown below)	465,996	459,079	678,500
Operating and administrative expenses	188,266	165,479	176,922
Operating and administrative expenses — related parties	8,366	9,326	12,125
Taxes other than income taxes	16,877	17,263	16,616
Depreciation	52,298	49,702	49,917
Amortization	3,418	3,090	2,629
Other income, net	(4,828)	(4,989)	(10,764)
Costs and expenses	730,393	698,950	925,945
Operating income	210,319	185,383	211,421
Interest expense	39,309	42,412	42,728
Income before income taxes	171,010	142,971	168,693
Income taxes	68,912	55,073	63,497
Net income	$ 102,098	$ 87,898	$ 105,196