Supplemental Disclosures of Cash Flow Information (Details) - USD ($) $ in Millions	3 Months Ended	9 Months Ended
Sep. 30, 2025	Sep. 30, 2024	Sep. 30, 2025
Supplemental Cash Flow Elements [Abstract]
Accounts Receivable, Sale	$ 263.3	$ 290.0
Increase (Decrease) in Finished Goods and Work in Process Inventories	(86.2)	101.3
Increase (Decrease) in Accounts Payable	(223.5)	(182.6)
Customer Advances and Deposits	4.6	32.8
Increase (Decrease) in Accrued Taxes Payable	7.3	(31.0)
Debt Instrument, Increase, Accrued Interest	40.9	11.0
Increase Decrease In Exchange Gas Receivable Payable		(161.6)	$ 59.8
Increase Decrease In Other Accruals	(20.4)	(17.8)
Increase Decrease In Prepayments And Other Current Assets	(45.4)	(61.2)
Employee Benefits and Share-Based Compensation	(26.4)	(1.1)
Adjustment to Reconcile Net Income to Cash Provided by (Used in) Operating Activity, Increase (Decrease) in Operating Capital	(26.0)	(85.8)
Dividends Payable, Current	$ 137.4	$ 125.3	$ 137.4

Increase or decrease in exchange gas receivable payable.

No definition available.

Name:	nix_IncreaseDecreaseInExchangeGasReceivablePayable
Namespace Prefix:	nix_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Increase decrease in other accruals.

+ References

No definition available.

+ Details

Name:	nix_IncreaseDecreaseInOtherAccruals
Namespace Prefix:	nix_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Increase decrease in prepayments and other current assets.

+ References

No definition available.

+ Details

Name:	nix_IncreaseDecreaseInPrepaymentsAndOtherCurrentAssets
Namespace Prefix:	nix_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Amount of decrease from sale of accounts receivable.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 326
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 11
-Subparagraph (i)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479319/326-20-50-11

+ Details

Name:	us-gaap_AccountsReceivableSale
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Carrying value as of the balance sheet date of dividends declared but unpaid on equity securities issued by the entity and outstanding. Used to reflect the current portion of the liabilities (due within one year or within the normal operating cycle if longer).

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Topic 210
-SubTopic 10
-Name Accounting Standards Codification
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02(20))
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147480566/210-10-S99-1

+ Details

Name:	us-gaap_DividendsPayableCurrent
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Amount of expense for employee benefit and equity-based compensation.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 220
-SubTopic 10
-Name Accounting Standards Codification
-Section S99
-Paragraph 2
-Subparagraph (SX 210.5-03(2))
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147483621/220-10-S99-2

+ Details

Name:	us-gaap_EmployeeBenefitsAndShareBasedCompensation
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

The increase (decrease) during the reporting period in the aggregate amount of liabilities incurred (and for which invoices have typically been received) and payable to vendors for goods and services received that are used in an entity's business.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Name Accounting Standards Codification
-Section 45
-Paragraph 28
-Subparagraph (a)
-SubTopic 10
-Topic 230
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147482740/230-10-45-28

+ Details

Name:	us-gaap_IncreaseDecreaseInAccountsPayable
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

The increase (decrease) during the reporting period of all taxes owed but not paid, including income, property and other taxes.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 230
-SubTopic 10
-Name Accounting Standards Codification
-Section 45
-Paragraph 28
-Subparagraph (a)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147482740/230-10-45-28

+ Details

Name:	us-gaap_IncreaseDecreaseInAccruedTaxesPayable
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

The increase (decrease) during the reporting period in the book value of finished goods inventory and work in process inventory.

+ References

+ Details

Name:	us-gaap_IncreaseDecreaseInFinishedGoodsAndWorkInProcessInventories
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Amount of increase (decrease) in asset and (increase) decrease in liability, used in operating activity in reconciling net income to reflect cash provided by (used in) operating activity when indirect cash flow method is applied.

+ References

+ Details

Name:	us-gaap_IncreaseDecreaseInOperatingCapital
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Increase for accrued, but unpaid interest on the credit facility for the period.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 235
-SubTopic 10
-Name Accounting Standards Codification
-Section S50
-Paragraph 1
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147480738/235-10-S50-1

Reference 2: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 235
-SubTopic 10
-Name Accounting Standards Codification
-Section S99
-Paragraph 1
-Subparagraph (SX 210.4-08(f))
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147480678/235-10-S99-1

+ Details

Name:	us-gaap_LineOfCreditFacilityIncreaseAccruedInterest
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Cash received from customers for deposits on goods or services during the period; excludes deposits with other institutions and time deposits, which pertain to financial services entities.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Topic 230
-SubTopic 10
-Name Accounting Standards Codification
-Section 45
-Paragraph 25
-Subparagraph (a)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147482740/230-10-45-25

+ Details

Name:	us-gaap_ProceedsFromDepositsFromCustomers
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- References

No definition available.

+ Details

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