Commitments and Guarantees (Details) - USD ($) $ in Millions	6 Months Ended	12 Months Ended
	Jun. 30, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Unrecorded Unconditional Purchase Obligation [Line Items]
percentage of output		25.00%
Capital Leased Assets, Gross		$ 57
Capital Leases, Lessee Balance Sheet, Assets by Major Class, Accumulated Depreciation		6
Capital Lease Obligations, Current		2
Purchase Commitment, Remaining Minimum Amount Committed		49
Deferred Costs, Leasing, Accumulated Amortization	$ 2	3	$ 1
Interest Expense, Lessee, Assets under Capital Lease	$ 3	4	2
Long-term Purchase Commitment, Amount		132
Jointly Owned Utility Plant, Ownership Amount of Construction Work in Progress		12
Operating Leases, Rent Expense		9	10	$ 0
Operating Leases, Future Minimum Payments Receivable, in Two Years		3
Operating Leases, Future Minimum Payments, Due in Three Years		3
Operating Leases, Future Minimum Payments Receivable, in Five Years		2
Contractual Obligation		318
Operating Leases, Rent Expense, Sublease Rentals		4	4	$ 3
Future Minimum Sublease Rentals, Sale Leaseback Transactions, within Three Years		3
Capital Addition Purchase Commitments [Member]
Unrecorded Unconditional Purchase Obligation [Line Items]
Jointly Owned Utility Plant, Ownership Amount of Construction Work in Progress		$ 108	$ 21