GOODWILL AND INTANGIBLE ASSETS (Details) - USD ($)	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019	Dec. 31, 2019
Goodwill and Intangible Assets [Line Items]
Goodwill	$ 950,000,000		$ 950,000,000		$ 950,000,000
Intangible assets					0
Accumulated amortization	44,000,000		44,000,000		42,000,000
Other intangible assets					0
Amortization expense	1,000,000	$ 1,000,000	2,000,000	$ 1,000,000
Expected amortization expense, year one	3,000,000		3,000,000
Expected amortization expense, year two	3,000,000		3,000,000
Expected amortization expense, year three	3,000,000		3,000,000
Expected amortization expense, year four	3,000,000		3,000,000
Expected amortization expense, year five	3,000,000		3,000,000
ITCTransmission [Member]
Goodwill and Intangible Assets [Line Items]
Goodwill	173,000,000		173,000,000		173,000,000
METC LLC [Member]
Goodwill and Intangible Assets [Line Items]
Goodwill	454,000,000		454,000,000		454,000,000
Intangible assets	17,000,000		17,000,000		19,000,000
Accumulated amortization	42,000,000		42,000,000		40,000,000
ITC Midwest LLC [Member]
Goodwill and Intangible Assets [Line Items]
Goodwill	323,000,000		323,000,000		323,000,000
ITC Great Plains LLC [Member]
Goodwill and Intangible Assets [Line Items]
Intangible assets	14,000,000		14,000,000		14,000,000
Accumulated amortization	$ 2,000,000		$ 2,000,000		$ 2,000,000