EARNINGS PER SHARE Schedule of Basic and Diluted Earnings Per Common Share (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
Numerator:
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
Less: dividends declared and paid — common and restricted shares																		108,211		95,503		84,104
Undistributed earnings																		$ 134,195		$ 148,580		$ 149,402
Percentage allocated to common shares	[4]																	99.30%		99.20%		99.10%
Numerator for basic and diluted earnings per common share																		$ 240,776		$ 242,215		$ 231,408
Denominator:
Basic earnings per common share — weighted average common shares outstanding																		153,670,087		155,363,848		155,736,384
Incremental shares for stock options, ESPP shares and performance shares — weighted average assumed conversion																		1,031,034		1,453,804		1,288,620
Diluted earnings per common share — adjusted weighted average shares and assumed conversion																		154,701,121		156,817,652		157,025,004
Per common share net income:
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
Percentage allocated to common shares:
Weighted average restricted shares (participating securities)																		1,102,051		1,277,128		1,472,967
Denominator for percentage allocated to common shares — total weighted average shares outstanding																		154,772,138		156,640,976		157,209,351
Common Shares
Numerator:
Less: dividends declared and paid — common and restricted shares																		$ 107,520		$ 94,824		$ 83,351
Undistributed earnings																		$ 133,256		$ 147,391		$ 148,057

[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.

[4] Weighted-average common shares outstanding153,670,087 155,363,848 155,736,384Weighted-average restricted shares and deferred stock units (participating securities)1,102,051 1,277,128 1,472,967 Total154,772,138 156,640,976 157,209,351 Percentage allocated to common shares99.3% 99.2% 99.1%