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Commitments and Contingencies (Narrative) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Other Commitments [Line Items]
Royalty Expense	$ 26	$ 25	$ 22
Guarantor Obligations, Current Carrying Value	0
LTSA [Member]
Unrecorded Unconditional Purchase Obligation
Unrecorded Unconditional Purchase Obligation	243
Electric Generation Equipment [Member]
Other Commitments [Line Items]
Operating Leases, Rent Expense, Net	43	41	38
Office Equipment [Member]
Other Commitments [Line Items]
Operating Leases, Rent Expense, Net	10	$ 9	$ 9
Operating Leases, Future Minimum Payments Due, Next Twelve Months	6
Unrecorded Unconditional Purchase Obligation
Operating Leases, Future Minimum Payments, Due in Two Years	6
Operating Leases, Future Minimum Payments, Due in Three Years	8
Operating Leases, Future Minimum Payments, Due in Four Years	8
Operating Leases, Future Minimum Payments, Due in Five Years	7
Operating Leases, Future Minimum Payments, Due Thereafter	18
Operating Leases, Future Minimum Payments Due	53
Minimum [Member] \| LTSA [Member]
Unrecorded Unconditional Purchase Obligation
Unrecorded Unconditional Purchase Obligation	$ 18
Unrecorded Unconditional Purchase Obligation, Term	1 year
Maximum [Member] \| LTSA [Member]
Unrecorded Unconditional Purchase Obligation
Unrecorded Unconditional Purchase Obligation, Term	19 years