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Revenue Recognition (Customer Accounts Receivable) (Details) - USD ($) $ in Millions	Sep. 30, 2021	Dec. 31, 2020
Revenue from Contract with Customer [Abstract]
Customer Accounts Receivable, Billed (Less Reserve)	$ 272.7	$ 400.0
Customer Accounts Receivable, Unbilled (Less Reserve)	$ 204.5	$ 327.2

X

- Definition

Customer Accounts Receivable, Billed (Less Reserve)

+ References

No definition available.

+ Details

Name:	nix_CustomerAccountsReceivableBilledLessReserve
Namespace Prefix:	nix_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

X

- Definition

Customer Accounts Receivable, Unbilled (Less Reserve)

+ References

No definition available.

+ Details

Name:	nix_CustomerAccountsReceivableUnbilledLessReserve
Namespace Prefix:	nix_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

X

- References

No definition available.

+ Details

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