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Property, Plant, and Equipment (Narrative) (Details) $ in Millions	Mar. 31, 2021 USD ($)
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