REGULATORY MATTERS (Details)	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2020 USD ($)	Jun. 30, 2019 USD ($)	Jun. 30, 2020 USD ($)	Jun. 30, 2019 USD ($)	Dec. 31, 2018 USD ($)	Jul. 16, 2020	May 21, 2020	Dec. 31, 2019 USD ($)	Jun. 11, 2019	Oct. 18, 2018	Apr. 20, 2018	Sep. 28, 2016
Regulatory Liabilities [Line Items]
Transmission Rate, Applicable Period			1 year
Revenue true-up amount reflected in customer bill, period of recognition			2 years
Revenue (increase) decrease	$ (342,000,000)	$ (320,000,000)	$ (664,000,000)	$ (627,000,000)
Current regulatory liability	$ 47,000,000		47,000,000					$ 123,000,000
Accrued interest			$ 3,000,000	$ 9,000,000
METC LLC [Member]
Regulatory Liabilities [Line Items]
Rate Of Return On Equity												12.38%
ITC Midwest LLC [Member]
Regulatory Liabilities [Line Items]
Rate Of Return On Equity												12.38%
MISO Operating Subsidiaries
Regulatory Liabilities [Line Items]
Incentive Adder for Independent Transmission Ownership	25		25							25
Basis Point Incentive Adder for RTO Participation	50		50
Accrued interest			$ (1,000,000)
ITC Great Plains LLC [Member]
Regulatory Liabilities [Line Items]
Current regulatory liability	$ 3,000,000		$ 3,000,000
Incentive Adder for Independent Transmission Ownership	100		100						100
Basis Point Incentive Adder for RTO Participation	50		50
Revised Rate Of Return On Equity	12.16%		12.16%
Minimum [Member] | MISO Operating Subsidiaries
Regulatory Liabilities [Line Items]
Incentive Adder for Independent Transmission Ownership											50
Minimum [Member] | ITC Great Plains LLC [Member]
Regulatory Liabilities [Line Items]
Revised Rate Of Return On Equity	10.66%		10.66%
Maximum [Member] | MISO Operating Subsidiaries
Regulatory Liabilities [Line Items]
Incentive Adder for Independent Transmission Ownership											100
Subsequent Event | ITC Great Plains LLC [Member]
Regulatory Liabilities [Line Items]
Incentive Adder for Independent Transmission Ownership						25
Rate of Return on Equity and Capital Structure Initial Complaint [Member]
Regulatory Liabilities [Line Items]
Required customer refund paid					$ 7,000,000
FERC Order [Member] | Rate of Return on Equity and Capital Structure Initial Complaint [Member] | Minimum [Member]
Regulatory Liabilities [Line Items]
Revised Rate Of Return On Equity	10.02%		10.02%				10.02%	9.88%				10.32%
FERC Order [Member] | Rate of Return on Equity and Capital Structure Initial Complaint [Member] | Maximum [Member]
Regulatory Liabilities [Line Items]
Revised Rate Of Return On Equity							12.62%	12.24%				11.35%