COMMITMENTS AND CONTINGENT LIABILITIES (Details) - USD ($)	3 Months Ended
	Mar. 31, 2016	Mar. 31, 2015	Dec. 22, 2016	Jan. 31, 2016	Dec. 31, 2015	Oct. 20, 2015	Apr. 06, 2015	Jan. 30, 2015	Jan. 05, 2015	Nov. 30, 2014
Commitments and Contingent Liabilities
Non-current regulatory liabilities	$ 273,564,000				$ 254,788,000
Operating revenues	280,133,000	$ 272,487,000
Interest expense	50,417,000	48,474,000
Net income	64,237,000	67,132,000
Estimated Potential Refund Related to Return on Equity Complaint
Commitments and Contingent Liabilities
Non-current regulatory liabilities	186,900,000				168,000,000
Rate of Return on Equity and Capital Structure Complaints
Commitments and Contingent Liabilities
Additional potential estimated refund, pre-tax	55,400,000
Additional potential estimated reduction of net income	34,000,000
Rate of Return on Equity and Capital Structure Complaints | Impact from Recognition of Liability
Commitments and Contingent Liabilities
Operating revenues	(17,500,000)	(7,500,000)
Interest expense	1,400,000	400,000
Net income	(11,500,000)	$ (4,800,000)
Rate of Return on Equity and Capital Structure Complaints | Minimum
Commitments and Contingent Liabilities
Estimated potential refund	186,900,000
Rate of Return on Equity and Capital Structure Complaints | Maximum
Commitments and Contingent Liabilities
Estimated potential refund	$ 242,300,000
Rate of Return on Equity and Capital Structure Complaints | Complaint One | Expert Witness Testimony
Commitments and Contingent Liabilities
Recommended ROE rate in the event FERC elects to change base ROE							11.39%
Presiding administrative law judges recommended base ROE on initial decision on initial complain			10.32%
Presiding administrative law judges recommended maximum ROE on initial decision on initial complain							11.35%
Recommended rate of return on equity in the event FERC elects to change base return on equity							12.38%
Rate of Return on Equity and Capital Structure Complaints | Complaint Two | Expert Witness Testimony
Commitments and Contingent Liabilities
Recommended ROE rate in the event FERC elects to change base ROE				10.96%		10.75%
Recommended rate of return on equity in the event FERC elects to change base return on equity						12.38%
ITCTransmission
Commitments and Contingent Liabilities
Rate of return on equity	12.38%
FERC approved capital structure, equity percentage	60.00%
ITCTransmission | Rate of Return on Equity and Capital Structure Complaints | Complaint One
Commitments and Contingent Liabilities
Reduced rate of return on equity	9.15%
Complaint capital structure, equity percentage	50.00%
ITCTransmission | Rate of Return on Equity and Capital Structure Complaints | Complaint Two
Commitments and Contingent Liabilities
Reduced rate of return on equity	8.67%
ITCTransmission | Sales and Use Tax Audit
Commitments and Contingent Liabilities
Potential liability	$ 18,600,000
Recorded contingent liability	8,800,000				$ 5,900,000
ITCTransmission | Sales and Use Tax Audit | Audit Period
Commitments and Contingent Liabilities
Potential liability	$ 10,400,000
METC
Commitments and Contingent Liabilities
Rate of return on equity	12.38%
FERC approved capital structure, equity percentage	60.00%
Approved incentive adder for RTO participation									0.50%
Requested incentive adder for RTO participation										0.50%
METC | Rate of Return on Equity and Capital Structure Complaints | Complaint One
Commitments and Contingent Liabilities
Reduced rate of return on equity	9.15%
Complaint capital structure, equity percentage	50.00%
METC | Rate of Return on Equity and Capital Structure Complaints | Complaint Two
Commitments and Contingent Liabilities
Reduced rate of return on equity	8.67%
METC | Sales and Use Tax Audit
Commitments and Contingent Liabilities
Potential liability	$ 10,400,000
Recorded contingent liability	$ 0
ITC Midwest
Commitments and Contingent Liabilities
Rate of return on equity	12.38%
FERC approved capital structure, equity percentage	60.00%
Approved incentive adder for RTO participation									0.50%
Requested incentive adder for RTO participation										0.50%
Requested incentive adder for independence								1.00%
Approved incentive adder for independence	0.50%
ITC Midwest | Rate of Return on Equity and Capital Structure Complaints | Complaint One
Commitments and Contingent Liabilities
Reduced rate of return on equity	9.15%
Complaint capital structure, equity percentage	50.00%
ITC Midwest | Rate of Return on Equity and Capital Structure Complaints | Complaint Two
Commitments and Contingent Liabilities
Reduced rate of return on equity	8.67%
MISO Operating Subsidiaries
Commitments and Contingent Liabilities
Equity in capital structure for ratemaking purposes	$ 3,000,000,000
Effect on net income from 10 basis point reduction in the authorized base return on equity	$ 3,000,000