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