Guarantees Mortgage Loans Repurchase Reserve Rollforward (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018	Dec. 31, 2018	Dec. 31, 2017
Guarantees [Abstract]
Reserve For Mortgage Loan Repurchase Losses	$ 24	$ 39	$ 26	$ 39
Mortgage Repurchase Reserve, Provision for Mortgage Loan Repurchase Losses	$ (2)	$ 0