Summary of Significant Accounting Policies (Details) - USD ($)	12 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2013
Property, Plant and Equipment
Goodwill impairments	$ 0	$ 0	$ 0
Impairment of long-lived assets	$ 0	$ 0	$ 0
Gas Utility
Property, Plant and Equipment
Depreciation expense as percentage of related average depreciable base	2.20%	2.30%	2.30%
Electric Utility
Property, Plant and Equipment
Depreciation expense as percentage of related average depreciable base	2.50%	2.50%	2.40%
Maximum
Property, Plant and Equipment
Maximum useful life of computer software	15 years
Operating Expense
Property, Plant and Equipment
Depreciation expense	$ 0
Removal Costs
Property, Plant and Equipment
Period to recover costs related to regulatory assets	5 years