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Property, Plant, and Equipment (Narrative) (Details) $ in Millions | Mar. 31, 2021 USD ($)
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Property, Plant and Equipment [Abstract] |
Plant and Equipment associated with Schahfer Generating Station Retirement | $ 861.6

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- Definition Plant and Equipment Associated with Schahfer Generating Station Retirement + References No definition available. + Details
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Namespace Prefix: | nix_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

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- References No definition available. + Details
Name: | us-gaap_PropertyPlantAndEquipmentAbstract
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration