Guarantees Repurchased Mortgage Loan (Details) - USD ($) $ in Millions	Mar. 31, 2018	Dec. 31, 2017
Repurchased mortgage loans, carrying value	$ 215	$ 219
Performing Financial Instruments [Member] | Loans Held-For-Investment [Member]
Repurchased mortgage loans, carrying value	197	203
Nonperforming Financing Receivable [Member] | Loans Held-For-Investment [Member]
Repurchased mortgage loans, carrying value	$ 18	$ 16