RETIREMENT BENEFITS AND ASSETS HELD IN TRUST Schedule of Net Defined Benefit Cost Components (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Defined Benefit Plans
Defined Benefit Plan Disclosure
Service cost	$ 2	$ 2
Interest cost	1	1
Expected return on plan assets	(1)	(1)
Net cost	2	2
Other Postretirement Benefits Plan
Defined Benefit Plan Disclosure
Service cost	0	0
Interest cost	0	0
Expected return on plan assets	0	0
Net cost	$ 0	$ 0