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Variable Interest Entities (Narrative) (Details) $ in Millions | 12 Months Ended
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Dec. 31, 2021 USD ($) Rate MW | Dec. 31, 2020 USD ($) | Dec. 31, 2019 USD ($)
Variable Interest Entity [Line Items] |  |  |
Wind Power Purchase Agreement, Purchase Percentage | Rate | 100.00% |  |
Payment to renewable generation asset developer | $ 240.4 | $ 85.3 | $ 0.0
Rosewater |  |  |
Variable Interest Entity [Line Items] |  |  |
Nameplate Capacity | MW | 102 |  |
Payment to renewable generation asset developer | $ 7.4 | 85.3 |
Indiana Crossroads Wind |  |  |
Variable Interest Entity [Line Items] |  |  |
Nameplate Capacity | MW | 302 |  |
Payment to renewable generation asset developer | $ 233.0 | $ 0.0 |

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- Definition Nameplate Capacity + References No definition available. + Details
Name: | nix_NameplateCapacity
Namespace Prefix: | nix_
Data Type: | dtr-types:powerItemType
Balance Type: | na
Period Type: | instant

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- Definition Payment to renewable generation asset developer + References No definition available. + Details
Name: | nix_PaymentToRenewableGenerationAssetDeveloper
Namespace Prefix: | nix_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

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- Definition Wind Power Purchase Agreement, Purchase Percentage + References No definition available. + Details
Name: | nix_WindPowerPurchaseAgreementPurchasePercentage
Namespace Prefix: | nix_
Data Type: | dtr-types:percentItemType
Balance Type: | na
Period Type: | instant

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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. + Details
Name: | us-gaap_VariableInterestEntityLineItems
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Details
Name: | dei_LegalEntityAxis=nix_RosewaterMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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- Details
Name: | dei_LegalEntityAxis=nix_IndianaCrossroadsWindMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |