Guarantees Repurchased Mortgage Loan (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Repurchased mortgage loans, carrying value	$ 219	$ 242
Performing Financial Instruments [Member] | Loans Held For Investment [Member]
Repurchased mortgage loans, carrying value	203	230
Nonperforming Financing Receivable [Member] | Loans Held For Investment [Member]
Repurchased mortgage loans, carrying value	$ 16	$ 12