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Revenue Recognition (Customer Accounts Receivable) (Details) - USD ($) $ in Millions | Jun. 30, 2021 | Dec. 31, 2020
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Revenue from Contract with Customer [Abstract] |  |
Customer Accounts Receivable, Billed (Less Reserve) | $ 323.7 | $ 400.0
Customer Accounts Receivable, Unbilled (Less Reserve) | $ 203.6 | $ 327.2

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- 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

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- 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

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- References No definition available. + Details
Name: | us-gaap_RevenueFromContractWithCustomerAbstract
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration