RETIREMENT BENEFITS AND ASSETS HELD IN TRUST Additional Information (Details) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016	Dec. 31, 2017
Other Pension Plan
Retirement Benefits Disclosure
Defined Benefit Plan, Plan Assets, Contributions by Employer	$ 4,000,000
Other Pension Plan | Expectation
Retirement Benefits Disclosure
Expected additional current year contribution					$ 0
Supplemental Retirement Plans
Retirement Benefits Disclosure
Defined Benefit Plan, Plan Assets, Contributions by Employer	14,000,000
Supplemental Retirement Plans | Expectation
Retirement Benefits Disclosure
Expected additional current year contribution					0
Other Postretirement Benefits Plan
Retirement Benefits Disclosure
Defined Benefit Plan, Plan Assets, Contributions by Employer	6,000,000
Other Postretirement Benefits Plan | Expectation
Retirement Benefits Disclosure
Expected additional current year contribution					$ 2,000,000
Defined Contribution Retirement Savings Plan
Retirement Benefits Disclosure
Plan cost	$ 1,000,000	$ 1,000,000	$ 3,000,000	$ 3,000,000