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Revenue Recognition (Customer Accounts Receivable) (Details) - USD ($) $ in Millions | 3 Months Ended |
--- | --- | ---
Mar. 31, 2020 | Dec. 31, 2019
Revenue from Contract with Customer [Abstract] |  |
Customer Accounts Receivable, Billed (Less Reserve) | $ 449.9 | $ 466.6
Customer Accounts Receivable, Unbilled (Less Reserve) | 233.5 | $ 346.6
Increase (Decrease) in Customer Accounts Receivable, Billed (Less Reserve) | (16.7) |
Increase (Decrease) in Customer Accounts Receivable, Unbilled (Less Reserve) | $ (113.1) |

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- Definition Customer Accounts Receivable, Billed (Less Reserve) + 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 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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- Definition Increase (Decrease) in Customer Accounts Receivable, Billed (Less Reserve) + References No definition available. + Details
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Namespace Prefix: | nix_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

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- Definition Increase (Decrease) in Customer Accounts Receivable, Unbilled (Less Reserve) + References No definition available. + Details
Name: | nix_IncreaseDecreaseinCustomerAccountsReceivableUnbilledLessReserve
Namespace Prefix: | nix_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

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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