REGULATORY MATTERS Additional Information (Details) - USD ($) $ in Thousands	3 Months Ended																12 Months Ended
	Dec. 31, 2015		Sep. 30, 2015	[1],[2]	Jun. 30, 2015	[1],[2]	Mar. 31, 2015	[1],[2]	Dec. 31, 2014		Sep. 30, 2014	[2]	Jun. 30, 2014	[2]	Mar. 31, 2014	[2]	Dec. 31, 2015		Dec. 31, 2014		Dec. 31, 2013
Regulatory Matters
Transmission rate, applicable period																	1 year
Revenue true-up amount reflected in customer bill, period of recognition																	2 years
Regulatory assets	$ 248,112								$ 229,105								$ 248,112		$ 229,105
Operating revenues	224,034	[1],[2]	$ 273,189		$ 275,058		$ 272,487		231,097	[2]	$ 270,134		$ 263,214		$ 258,603		1,044,768	[1],[2]	1,023,048	[2]	$ 941,272
Interest expense																	203,779		186,636		168,319
Net income	37,365	[1],[2]	$ 65,573		$ 72,336		$ 67,132		$ 46,738	[2],[3]	$ 73,873	[3]	$ 54,336	[3]	$ 69,136	[3]	242,406	[1],[2]	$ 244,083	[2],[3]	$ 233,506
MISO Formula Rate Template Modification Filing | Impact from Recognition of Liability
Regulatory Matters
Operating revenues																	9,500
Interest expense																	900
Net income																	6,200
ITC Great Plains | Start-up, Development and Pre-construction Expenses, Including Associated Carrying Charges
Regulatory Matters
Regulatory assets	12,600																12,600
Regulatory assets, accumulated amortization	1,000																$ 1,000
Regulatory asset, amortization period																	10 years
Equity component of carrying charges	$ 9,900																$ 9,900
ITC Midwest
Regulatory Matters
Rate discount																	$ 4,100
Rate discount period																	8 years

[1] During the third and fourth quarters of 2015, we recognized an aggregate regulatory liability for the refund relating to the formula rate template modifications filing as described in Note 4, which resulted in a reduction in operating revenues and operating income of $9.5 million and an estimated $6.2 million reduction to net income for the year ended December 31, 2015.

[2] During the year ended December 31, 2015 and the fourth quarter of 2014, we recognized an aggregate estimated regulatory liability for the potential refunds relating to the ROE complaints as described in Note 16, which resulted in a reduction in operating revenues and operating income of $115.1 million and $46.9 million and an estimated $73.2 million and $28.9 million reduction to net income for the years ended December 31, 2015 and 2014, respectively.

[3] During the second quarter of 2014, we incurred a loss on extinguishment of debt of $29.2 million related to the tender of ITC Holdings Senior Notes as described in Note 8, which resulted in an estimated reduction to net income of $18.2 million.