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v3.21.2

Subsequent Event (Narrative) (Details) - USD ($) $ in Millions | Oct. 01, 2021 | Sep. 30, 2021
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Subsequent Event [Line Items] |  |
Plant and Equipment associated with Schahfer Generating Station Retirement |  | $ 831.8
Subsequent Event | Units 14 and 15 |  |
Subsequent Event [Line Items] |  |
Plant and Equipment associated with Schahfer Generating Station Retirement | $ 600.0 |

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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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- Definition Detail information of subsequent event by type. User is expected to use existing line items from elsewhere in the taxonomy as the primary line items for this disclosure, which is further associated with dimension and member elements pertaining to a subsequent event. + References No definition available. + Details
Name: | us-gaap_SubsequentEventLineItems
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Details
Name: | us-gaap_SubsequentEventTypeAxis=us-gaap_SubsequentEventMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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- Details
Name: | nix_GenerationRetirementUnitsAxis=nix_Units14And15Member
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |