GOODWILL AND INTANGIBLE ASSETS (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2016	Mar. 31, 2015	Dec. 31, 2015
Goodwill and Intangible Assets
Goodwill	$ 950,163		$ 950,163
Intangible assets	44,950		45,602
Accumulated amortization	29,073		28,242
Amortization expense	800	$ 800
Expected amortization expense, year one	3,300
Expected amortization expense, year two	3,300
Expected amortization expense, year three	3,300
Expected amortization expense, year four	3,300
Expected amortization expense, year five	3,300
ITCTransmission
Goodwill and Intangible Assets
Goodwill	173,400		173,400
METC
Goodwill and Intangible Assets
Goodwill	453,800		453,800
Intangible assets	30,400		31,200
Accumulated amortization	28,000		27,200
ITC Midwest
Goodwill and Intangible Assets
Goodwill	323,000		323,000
ITC Great Plains
Goodwill and Intangible Assets
Intangible assets	14,500		14,400
Accumulated amortization	$ 1,100		$ 1,000