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Regulatory Matters (Schedule of COVID-19 Regulatory Impact) (Details) - USD ($) $ in Millions	Dec. 31, 2023	Dec. 31, 2022
Regulatory Matters COVID [Line Items]
COVID-19 related regulatory asset	$ 2,460.2	$ 2,580.8

X

- Definition

Regulatory Matters COVID [Line Items]

+ References

No definition available.

+ Details

Name:	nix_RegulatoryMattersCOVIDLineItems
Namespace Prefix:	nix_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

X

- Definition

The amount for the individual regulatory asset as itemized in a table of regulatory assets as of the end of the period.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Topic 980
-SubTopic 340
-Name Accounting Standards Codification
-Section 50
-Paragraph 1
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147481878/980-340-50-1

+ Details

Name:	us-gaap_RegulatoryAssets
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant