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Summary of Significant Accounting Policies - Income Taxes (Details) - USD ($) $ in Millions | 12 Months Ended
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Sep. 30, 2019 | Sep. 30, 2018 | Sep. 30, 2017
Accounting Policies [Abstract] |  |  |
Accumulated deferred investment tax credit | $ 2.3 | $ 2.6 |
Effective income tax rate reconciliation, at federal statutory income tax rate, percent | 21.00% | 24.50% | 35.00%

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- Definition The noncurrent portion of the reserve for accumulated deferred investment tax credits as of the balance sheet date. This is the remaining investment credit, which will reduce the cost of services collected from ratepayers by a ratable portion over the investment's regulatory life. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 255 -SubTopic 10 -Section 55 -Paragraph 8 -URI http://asc.fasb.org/extlink&oid=82838779&loc=d3e2408-110839 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.26(b)) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 740 -SubTopic 10 -Section 45 -Paragraph 27 -URI http://asc.fasb.org/extlink&oid=116821951&loc=d3e32262-109318 + Details
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