SUPPLEMENTARY QUARTERLY FINANCIAL INFORMATION (Unaudited) (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended																12 Months Ended
	Dec. 31, 2015		Sep. 30, 2015		Jun. 30, 2015		Mar. 31, 2015		Dec. 31, 2014		Sep. 30, 2014		Jun. 30, 2014		Mar. 31, 2014		Dec. 31, 2015		Dec. 31, 2014		Dec. 31, 2013
Selected Quarterly Financial Information [Abstract]
Operating revenues	$ 224,034	[1],[2]	$ 273,189	[1],[2]	$ 275,058	[1],[2]	$ 272,487	[1],[2]	$ 231,097	[2]	$ 270,134	[2]	$ 263,214	[2]	$ 258,603	[2]	$ 1,044,768	[1],[2]	$ 1,023,048	[2]	$ 941,272
Operating income	103,213	[1],[2]	149,644	[1],[2]	158,408	[1],[2]	149,452	[1],[2]	119,028	[2]	161,432	[2]	158,928	[2]	153,441	[2]	560,717	[1],[2]	592,829	[2]	496,061
NET INCOME	$ 37,365	[1],[2]	$ 65,573	[1],[2]	$ 72,336	[1],[2]	$ 67,132	[1],[2]	$ 46,738	[2],[3]	$ 73,873	[2],[3]	$ 54,336	[2],[3]	$ 69,136	[2],[3]	$ 242,406	[1],[2]	$ 244,083	[2],[3]	$ 233,506
Basic	$ 0.25		$ 0.42		$ 0.47		$ 0.43		$ 0.30		$ 0.47		$ 0.34		$ 0.44		$ 1.57		$ 1.56		$ 1.49
Diluted	$ 0.24		$ 0.42		$ 0.46		$ 0.43		$ 0.30		$ 0.47		$ 0.34		$ 0.43		$ 1.56		$ 1.54		$ 1.47
Loss on extinguishment of debt																	$ 0		$ (29,205)		$ 0
Loss on Extinguishment of Debt
Terminated Entergy Transaction
Reduction of net income																	$ 18,200
Rate of Return on Equity and Capital Structure Complaint
Selected Quarterly Financial Information [Abstract]
Operating revenues																			46,900
Terminated Entergy Transaction
Reduction of net income																			$ 28,900

[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.