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Revenue Recognition (Customer Accounts Receivable) (Details) - USD ($) $ in Millions | 12 Months Ended |
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Dec. 31, 2018 | Dec. 31, 2017
Revenue Recognition [Abstract] |  |  |
Customer Accounts Receivable, Billed (Less Reserve) | [1] | $ 540.5 | $ 477.0
Customer Accounts Receivable, Unbilled (Less Reserve) | [2] | 349.1 | $ 378.6
Increase (Decrease) in Customer Accounts Receivable, Billed (Less Reserve) | [1] | 63.5 |
Increase (Decrease) in Customer Accounts Receivable, Unbilled (Less Reserve) | [2] | $ (29.5) |
[1] | Customer billed receivables increased over the period due to November 2018 being colder than November 2017, leading to more gas usage included in December bills.
[2] | Customer unbilled receivables decreased over the period due December 2018 being warmer than December 2017, leading to less estimated gas usage.

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