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RETIREMENT BENEFITS AND ASSETS HELD IN TRUST Effect of One-Percentage-Point Change in Assumed Health Care Cost Trend Rates (Details) - Other Postretirement Benefits Plan
$ in Thousands
12 Months Ended
Dec. 31, 2015
USD ($)
Defined Benefit Plan Disclosure  
Effect on total of service and interest cost, one percentage point increase $ 3,288
Effect on total of service and interest cost, one percentage point decrease (2,282)
Effect on postretirement benefit obligation, one percentage point increase 13,452
Effect on postretirement benefit obligation, one percentage point decrease $ (9,869)