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Property, Plant, and Equipment (Narrative) (Details) $ in Millions	6 Months Ended
	Jun. 30, 2021 USD ($)
Property, Plant and Equipment [Line Items]
Charges associated with Schahfer Generating Station Retirement	$ 8.6
Plant and Equipment associated with Schahfer Generating Station Retirement	$ 823.1

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- Definition

Charges associated with Schahfer Generating Station Retirement

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No definition available.

+ Details

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Namespace Prefix:	nix_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

X

- 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

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.

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No definition available.

+ Details

Name:	us-gaap_PropertyPlantAndEquipmentLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration