Pension and Other Post-Employment Benefits (Defined Periodic Pension And OPEB Expense) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Pension Benefit [Member]
Defined Benefit Plan Disclosure [Line Items]
Current service cost	$ 1	$ 2	$ 2
Interest cost	7	8	9
Expected return on plan assets	(9)	(11)	(12)
Amortization of net actuarial gains and losses	1		2
Total Defined Benefit Plan Expense		(1)	1
OPEB [Member]
Defined Benefit Plan Disclosure [Line Items]
Current service cost	8	10	10
Interest cost	3	4	4
Amortization of net actuarial gains and losses	(1)	(1)
Amortization of net prior service costs	(1)
Curtailment	(1)
Curtailment	(5)
Total Defined Benefit Plan Expense	$ 3	$ 13	$ 14