Loss And Loss Adjustment Expense Reserves (Loss And LAE Activity) (Narrative) (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017
Loss And Loss Adjustment Expense Reserves [Line Items]
Weighted average risk-free rate used to discount claim liability	2.67%
Reinsurance Receivables Case Basis	$ 20		$ 17
Loss and loss adjustment expense reserves	1,006		979
Losses and loss adjustment	72	$ 94
Second Lien Residential Mortgage Backed Securities And Reserves [Member]
Loss And Loss Adjustment Expense Reserves [Line Items]
Losses and loss adjustment	2	23
Lae [Member]
Loss And Loss Adjustment Expense Reserves [Line Items]
Loss and loss adjustment expense reserves	74		$ 66
Losses and loss adjustment	13	$ 12
Ineligible Mortgage Loans [Member]
Loss And Loss Adjustment Expense Reserves [Line Items]
Contract Claims On Ineligible Mortgage Loans Inception To Date Incurred Loss On Transactions	$ 436