RETIREMENT BENEFITS AND ASSETS HELD IN TRUST Plan Assets By Category (Details)	Dec. 31, 2015	Dec. 31, 2014
Retirement Plan
Defined Benefit Plan Disclosure
Asset allocation	100.00%	100.00%
Other Postretirement Benefits Plan
Defined Benefit Plan Disclosure
Asset allocation	100.00%	100.00%
Fixed income securities | Retirement Plan
Defined Benefit Plan Disclosure
Asset allocation	50.40%	48.80%
Fixed income securities | Other Postretirement Benefits Plan
Defined Benefit Plan Disclosure
Asset allocation	50.00%	57.20%
Equity securities | Retirement Plan
Defined Benefit Plan Disclosure
Asset allocation	49.60%	51.20%
Equity securities | Other Postretirement Benefits Plan
Defined Benefit Plan Disclosure
Asset allocation	50.00%	42.80%