Summary of Significant Accounting Policies - Average Composite Depreciation Rates (Details)	12 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2015
Public Utility, Property, Plant and Equipment [Line Items]
Amortization period	5 years
Gas Utility
Public Utility, Property, Plant and Equipment [Line Items]
Average composite depreciation rates	2.20%	2.20%	2.20%
Electric Utility
Public Utility, Property, Plant and Equipment [Line Items]
Average composite depreciation rates	2.40%	2.50%	2.50%