Commitments and Guarantees Lease Obligations (Details) - USD ($) $ in Millions	6 Months Ended	12 Months Ended
	Jun. 30, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Long-term Purchase Commitment [Line Items]
Operating Leases, Future Minimum Payments Due, Future Minimum Sublease Rentals		$ 4
Operating Leases, Rent Expense, Sublease Rentals		4	$ 4	$ 3
Capital Leases, Future Minimum Payments Due, Next Twelve Months		7
Contractual Obligation, Due in Next Fiscal Year		0
Operating Leases, Future Minimum Payments Due, Next Twelve Months		9
Capital Leases, Future Minimum Payments Due in Two Years		6
Contractual Obligation, Due in Second Year		15
Operating Leases, Future Minimum Payments, Due in Two Years		8
Capital Leases, Future Minimum Payments Due in Three Years		6
Contractual Obligation, Due in Third Year		15
Operating Leases, Future Minimum Payments, Due in Three Years		6
Capital Leases, Future Minimum Payments Due in Four Years		6
Contractual Obligation, Due in Fourth Year		14
Operating Leases, Future Minimum Payments, Due in Four Years		6
Capital Leases, Future Minimum Payments Due in Five Years		5
Contractual Obligation, Due in Fifth Year		14
Operating Leases, Future Minimum Payments, Due in Five Years		8
Capital Leases, Future Minimum Payments Due Thereafter		72
Contractual Obligation, Due after Fifth Year		260
Operating Leases, Future Minimum Payments, Due Thereafter		165
Capital Leases, Future Minimum Payments Due		102
Contractual Obligation		318
Operating Leases, Future Minimum Payments Due		202
Capital Leases, Future Minimum Payments, Interest Included in Payments		51
Capital Leases, Future Minimum Payments, Present Value of Net Minimum Payments		51
Deferred Costs, Leasing, Accumulated Amortization	$ 2	3	1
Interest Expense, Lessee, Assets under Capital Lease	$ 3	$ 4	$ 2