The expense recognized in the current period that allocates the cost of nonproduction tangible assets over their useful lives.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 360
-SubTopic 10
-Section 50
-Paragraph 1
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=6391035&loc=d3e2868-110229

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 28
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=98514028&loc=d3e3602-108585

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- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

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- Definition

Composite depreciation rate for public utility plants in service.

+ References

No definition available.

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- Definition

Amortization period for the recovery of regulatory asset, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

+ References

No definition available.

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Summary of Significant Accounting Policies - Property, Plant and Equipment and Related Depreciation (Details) - USD ($)	12 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2016
Property, Plant and Equipment
Depreciation expense	$ 0
Removal Costs
Property, Plant and Equipment
Amortization period	5 years
Gas Utility
Property, Plant and Equipment
Average composite depreciation rates	2.30%	2.20%	2.20%
Electric Utility
Property, Plant and Equipment
Average composite depreciation rates	2.20%	2.40%	2.50%