COMMITMENTS AND CONTINGENT LIABILITIES (Details) (USD $) In Millions, unless otherwise specified	Sep. 30, 2013	Dec. 31, 2012
FERC Audit Refund and Related Interest
Commitments and Contingent Liabilities
Regulatory liability	$ 13.0	$ 12.7
ITCTransmission | Sales and Use Tax
Commitments and Contingent Liabilities
Potential liability	18.0
ITCTransmission | Sales and Use Tax | Audit Period
Commitments and Contingent Liabilities
Potential liability	3.8
METC | Sales and Use Tax
Commitments and Contingent Liabilities
Potential liability	$ 11.2
ITC Midwest
Commitments and Contingent Liabilities
Rate of return on actual equity	12.38%
Reduced rate of return on actual equity	10.39%