RETIREMENT BENEFITS AND ASSETS HELD IN TRUST Additional Information (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Mar. 31, 2017	Mar. 31, 2016	Dec. 31, 2017
Defined Contribution Plan
Retirement Benefits Disclosure
Plan cost	$ 1	$ 2
Other Pension Plan | Expectation
Retirement Benefits Disclosure
Expected additional current year contribution			$ 4
Supplemental Retirement Plans | Expectation
Retirement Benefits Disclosure
Expected additional current year contribution			14
Other Postretirement Benefits Plan | Expectation
Retirement Benefits Disclosure
Expected additional current year contribution			$ 8