REGULATORY MATTERS Schedule of Regulatory Assets and Liabilities (Details) - USD ($) $ in Millions	Mar. 31, 2026	Dec. 31, 2025
Schedule of Regulatory Assets and Liabilities [Line Items]
Current regulatory assets	$ 29	$ 28
Non-current regulatory assets	238	197
Current regulatory liabilities	(18)	(23)
Non-current regulatory liabilities	(758)	(759)
Net regulatory assets	60	23
Revenue Deferrals, Including Accrued Interest
Schedule of Regulatory Assets and Liabilities [Line Items]
Current regulatory liabilities	(17)	(16)
Non-current regulatory liabilities	(20)	(22)
Revenue Accruals, Including Accrued Interest
Schedule of Regulatory Assets and Liabilities [Line Items]
Current regulatory assets	29	28
Non-current regulatory assets	$ 68	$ 33

Net Regulatory Asset (Liability) Associated with Revenue Accruals and Deferrals

No definition available.

Name:	itc_NetRegulatoryAssetLiabilityAssociatedwithRevenueAccrualsandDeferrals
Namespace Prefix:	itc_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

[Line Items] for Schedule of Regulatory Assets and Liabilities [Table]

+ References

No definition available.

+ Details

Name:	itc_ScheduleofRegulatoryAssetsandLiabilitiesLineItems
Namespace Prefix:	itc_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Carrying amount as of the balance sheet date of capitalized costs of regulated entities that are expected to be recovered through revenue sources within one year or the normal operating cycle, if longer. Such costs are capitalized if they meet both of the following criteria: a. It is probable that future revenue in an amount at least equal to the capitalized cost will result from inclusion of that cost in allowable costs for rate-making purposes. b. Based on available evidence, the future revenue will be provided to permit recovery of the previously incurred cost rather than to provide for expected levels of similar future costs. If the revenue will be provided through an automatic rate-adjustment clause, this criterion requires that the regulator's intent clearly be to permit recovery of the previously incurred cost.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Topic 980
-SubTopic 340
-Name Accounting Standards Codification
-Section 50
-Paragraph 1
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147478742/980-340-50-1

+ Details

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

- Definition

Carrying amount as of the balance sheet date of capitalized costs of regulated entities that are not expected to be recovered through revenue sources within one year or the normal operating cycle if longer.

+ References

+ Details

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

- Definition

The amount for the individual regulatory current liability as itemized in a table of regulatory current liabilities as of the end of the period.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Topic 980
-SubTopic 405
-Name Accounting Standards Codification
-Section 25
-Paragraph 1
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147477571/980-405-25-1

+ Details

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

- Definition

The amount for the individual regulatory noncurrent liability as itemized in a table of regulatory noncurrent liabilities as of the end of the period.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 210
-SubTopic 10
-Name Accounting Standards Codification
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02(24))
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147480566/210-10-S99-1

+ Details

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

- Details

Name:	us-gaap_RegulatoryLiabilityAxis=itc_RevenueDeferralsIncludingAccruedInterestMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_RegulatoryAssetAxis=itc_RevenueAccrualsIncludingAccruedInterestMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: