GOODWILL AND INTANGIBLE ASSETS (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015	Dec. 31, 2015
Goodwill and Intangible Assets
Goodwill	$ 950,163		$ 950,163		$ 950,163
Intangible assets	44,283		44,283		45,602
Accumulated amortization	29,905		29,905		28,242
Amortization expense	900	$ 900	1,700	$ 1,700
Expected amortization expense, year one	3,300		3,300
Expected amortization expense, year two	3,300		3,300
Expected amortization expense, year three	3,300		3,300
Expected amortization expense, year four	3,300		3,300
Expected amortization expense, year five	3,300		3,300
ITCTransmission
Goodwill and Intangible Assets
Goodwill	173,400		173,400		173,400
METC
Goodwill and Intangible Assets
Goodwill	453,800		453,800		453,800
Intangible assets	29,700		29,700		31,200
Accumulated amortization	28,700		28,700		27,200
ITC Midwest
Goodwill and Intangible Assets
Goodwill	323,000		323,000		323,000
ITC Great Plains
Goodwill and Intangible Assets
Intangible assets	14,600		14,600		14,400
Accumulated amortization	$ 1,200		$ 1,200		$ 1,000