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Gas in Storage (Details) - USD ($) $ in Millions	Mar. 31, 2020	Dec. 31, 2019
Gas in Storage [Abstract]
LIFO Inventory Amount	$ 29.9	$ 0.0

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- Definition

Gas in Storage [Abstract]

+ References

No definition available.

+ Details

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

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- Definition

The amount of LIFO (last in first out) inventory present at the reporting date when inventory is also valued using different valuation methods.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section 50
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=6361739&loc=d3e7789-107766

+ Details

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