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RETIREMENT BENEFITS AND ASSETS HELD IN TRUST Assumptions Used (Details)
12 Months Ended
Dec. 31, 2015
Dec. 31, 2014
Dec. 31, 2013
Defined Benefit Plans      
Benefit Obligation Assumptions      
Annual rate of salary increases 4.00% 4.00%  
Benefit Cost Assumptions      
Annual rate of salary increases 4.00%    
Expected long-term rate of return on plan assets 6.70% 6.75% 7.00%
Defined Benefit Plans | Minimum      
Benefit Obligation Assumptions      
Discount rate 4.01% 3.75% 4.60%
Annual rate of salary increases     4.00%
Benefit Cost Assumptions      
Discount rate 3.75% 4.60% 3.70%
Annual rate of salary increases   4.00% 5.00%
Defined Benefit Plans | Maximum      
Benefit Obligation Assumptions      
Discount rate 4.44% 4.05% 5.10%
Annual rate of salary increases     6.00%
Benefit Cost Assumptions      
Discount rate 4.05% 5.10% 4.45%
Annual rate of salary increases   6.00% 6.00%
Other Postretirement Benefits Plan      
Benefit Obligation Assumptions      
Discount rate 4.62% 4.20% 5.15%
Annual rate of salary increases 4.00% 4.00% 4.00%
Benefit Cost Assumptions      
Discount rate 4.20% 5.15% 4.20%
Annual rate of salary increases 4.00% 4.00% 5.00%
Other Postretirement Benefits Plan | Benefit Obligation      
Assumed Health Care Cost Trend Rates      
Health care cost trend rate assumed for next year 7.15% 7.25% 7.50%
Rate to which the cost trend rate is assumed to decline 5.00% 5.00% 5.00%
Year that the rate reaches the ultimate trend rate 2022 2022 2022
Annual rate of increase in dental benefit costs 5.00% 5.00% 5.00%
Other Postretirement Benefits Plan | Benefit Cost      
Benefit Cost Assumptions      
Expected long-term rate of return on plan assets 5.20% 5.50% 7.00%
Assumed Health Care Cost Trend Rates      
Health care cost trend rate assumed for next year 7.25% 7.50% 8.00%
Rate to which the cost trend rate is assumed to decline 5.00% 5.00% 5.00%
Year that the rate reaches the ultimate trend rate 2022 2022 2017