Regulatory Assets and Liabilities and Regulatory Matters (Details) - USD ($) $ in Thousands	12 Months Ended
	Sep. 30, 2015	Sep. 30, 2014
Regulatory Assets
Amortization period for remediation and investigation costs expected to be recovered	5 years
Unrealized gains (losses) on derivative financial instruments contracts	$ (3,262)	$ (1,363)
Fair value of electric utility electricity supply contracts	$ (533)	$ 345
Removal Costs
Regulatory Assets
Period to recover costs related to regulatory assets	5 years
Minimum
Regulatory Assets
Average remaining depreciable lives of the associated property	1 year
Minimum | Other Regulatory Assets
Regulatory Assets
Period to recover costs related to regulatory assets	1 year
Maximum
Regulatory Assets
Average remaining depreciable lives of the associated property	65 years
Maximum | Other Regulatory Assets
Regulatory Assets
Period to recover costs related to regulatory assets	20 years