COMMITMENTS AND CONTINGENT LIABILITIES (Details) $ in Millions	3 Months Ended				6 Months Ended		12 Months Ended	51 Months Ended
	Jun. 30, 2020 USD ($)	Mar. 31, 2020 USD ($)	Jun. 30, 2019 USD ($)	Mar. 31, 2019 USD ($)	Jun. 30, 2020 USD ($)	Jun. 30, 2019 USD ($)	Dec. 31, 2018 USD ($)	Dec. 31, 2022 USD ($)	May 21, 2020	Dec. 31, 2019 USD ($)	Sep. 30, 2016	Sep. 28, 2016	Feb. 12, 2015	Nov. 12, 2013
Commitments and Contingent Liabilities [Line Items]
Current regulatory liability	$ 47				$ 47					$ 123
Revenue (increase) decrease	(342)		$ (320)		(664)	$ (627)
Interest expense (decrease) increase	57		64		116	123
Estimated net income increase (reduction)	(121)	$ (92)	(87)	$ (84)	(213)	(171)
Estimated Potential Refund Related to Return on Equity Complaints
Commitments and Contingent Liabilities [Line Items]
Current regulatory liability	$ 16				$ 16		$ 151			$ 70
Rate of Return on Equity and Capital Structure Initial Complaint [Member]
Commitments and Contingent Liabilities [Line Items]
Required customer refund paid							$ 7
Rate of Return on Equity and Capital Structure Initial Complaint [Member] | FERC Order [Member] | Minimum [Member]
Commitments and Contingent Liabilities [Line Items]
Revised Rate Of Return On Equity	10.02%				10.02%				10.02%	9.88%		10.32%
Rate of Return on Equity and Capital Structure Initial Complaint [Member] | FERC Order [Member] | Maximum [Member]
Commitments and Contingent Liabilities [Line Items]
Revised Rate Of Return On Equity									12.62%	12.24%		11.35%
Rate of Return on Equity and Capital Structure Second Complaint | Presiding Administrative Law Judge Initial Decision | Minimum [Member]
Commitments and Contingent Liabilities [Line Items]
Recommended rate of return on equity											9.70%
Rate of Return on Equity and Capital Structure Second Complaint | Presiding Administrative Law Judge Initial Decision | Maximum [Member]
Commitments and Contingent Liabilities [Line Items]
Recommended rate of return on equity											10.68%
Rate of Return on Equity and Capital Structure Complaints | Impact from Recognition of Liability
Commitments and Contingent Liabilities [Line Items]
Revenue (increase) decrease	$ (32)		0		$ (32)	0
Interest expense (decrease) increase			2			4
Estimated net income increase (reduction)	26		$ (2)		25	$ (3)
Interest Income, Other	(3)				(3)
ITCTransmission [Member]
Commitments and Contingent Liabilities [Line Items]
Rate Of Return On Equity												12.38%
ITCTransmission [Member] | Rate of Return on Equity and Capital Structure Initial Complaint [Member]
Commitments and Contingent Liabilities [Line Items]
Reduced rate of return on equity														9.15%
ITCTransmission [Member] | Rate of Return on Equity and Capital Structure Second Complaint
Commitments and Contingent Liabilities [Line Items]
Reduced rate of return on equity													8.67%
METC LLC [Member]
Commitments and Contingent Liabilities [Line Items]
Rate Of Return On Equity												12.38%
METC LLC [Member] | Rate of Return on Equity and Capital Structure Initial Complaint [Member]
Commitments and Contingent Liabilities [Line Items]
Reduced rate of return on equity														9.15%
METC LLC [Member] | Rate of Return on Equity and Capital Structure Second Complaint
Commitments and Contingent Liabilities [Line Items]
Reduced rate of return on equity													8.67%
ITC Midwest LLC [Member]
Commitments and Contingent Liabilities [Line Items]
Rate Of Return On Equity												12.38%
ITC Midwest LLC [Member] | Rate of Return on Equity and Capital Structure Initial Complaint [Member]
Commitments and Contingent Liabilities [Line Items]
Reduced rate of return on equity														9.15%
ITC Midwest LLC [Member] | Rate of Return on Equity and Capital Structure Second Complaint
Commitments and Contingent Liabilities [Line Items]
Reduced rate of return on equity													8.67%
MISO Operating Subsidiaries
Commitments and Contingent Liabilities [Line Items]
Customer Refund Liability, Current					20
Equity in capital structure for ratemaking purposes	$ 5,000				$ 5,000
Incentive Adder for Independent Transmission Ownership	10				10
Effect on net income from 10 basis point reduction in the authorized base return on equity	$ 5				$ 5
Basis Point Incentive Adder for RTO Participation	50				50
Forecast [Member]
Commitments and Contingent Liabilities [Line Items]
Future Development Payments, Reasonably Possible Estimate								$ 120