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Regulatory Matters (Schedule of COVID-19 Regulatory Impact) (Details) - USD ($) $ in Millions | Dec. 31, 2023 | Dec. 31, 2022
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Regulatory Matters COVID [Line Items] |  |
COVID-19 related regulatory asset | $ 2,460.2 | $ 2,580.8

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- Definition Regulatory Matters COVID [Line Items] + References No definition available. + Details
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Namespace Prefix: | nix_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Definition The amount for the individual regulatory asset as itemized in a table of regulatory assets as of the end of the period. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Topic 980 -SubTopic 340 -Name Accounting Standards Codification -Section 50 -Paragraph 1 -Publisher FASB -URI https://asc.fasb.org//1943274/2147481878/980-340-50-1 + Details
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Balance Type: | debit
Period Type: | instant