Guarantees and Commitments Guarantees and Commitments (Schedule of Lease Commitments) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Guarantees and Commitments [Abstract]
Operating Leases, Future Minimum Payments Due, Next Twelve Months	$ 35
Operating Leases, Future Minimum Payments, Due in Two Years	35
Operating Leases, Future Minimum Payments, Due in Three Years	34
Operating Leases, Future Minimum Payments, Due in Four Years	36
Operating Leases, Future Minimum Payments, Due in Five Years	33
Operating Leases, Future Minimum Payments, Due Thereafter	316
Operating Leases, Future Minimum Payments Due	489
Operating Leases, Rent Expense	$ 46	$ 51	$ 51