RETIREMENT BENEFITS AND ASSETS HELD IN TRUST Schedule of Net Defined Benefit Cost Components (Details) - USD ($)	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Defined Benefit Plan Disclosure [Line Items]
Defined Contribution Plan, Cost	$ 1,000,000	$ 1,000,000	$ 4,000,000	$ 3,000,000
Pension plans
Defined Benefit Plan Disclosure [Line Items]
Service cost	2,000,000	2,000,000	4,000,000	4,000,000
Interest cost	1,000,000	1,000,000	2,000,000	2,000,000
Expected return on plan assets	(1,000,000)	(1,000,000)	(2,000,000)	(2,000,000)
Net cost	2,000,000	2,000,000	4,000,000	4,000,000
Other Postretirement Benefits Plan
Defined Benefit Plan Disclosure [Line Items]
Service cost	2,000,000	3,000,000	5,000,000	5,000,000
Interest cost	1,000,000	1,000,000	2,000,000	2,000,000
Expected return on plan assets	(1,000,000)	(1,000,000)	(2,000,000)	(2,000,000)
Net cost	$ 2,000,000	$ 3,000,000	$ 5,000,000	$ 5,000,000