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Mortgage-Backed Securities - OTTI - Significant Inputs and Assumptions (Detail) - Non-agency MBS
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Weighted Average    
Investment Holdings [Line Items]    
Loss Severity 64.00% 58.00%
60 days delinquent 21.00% 21.00%
Credit Enhancement 16.00% 25.00%
3 Month CPR 10.00% 7.00%
12 Month CPR 11.00% 6.00%
Minimum    
Investment Holdings [Line Items]    
Loss Severity 28.00% 25.00%
60 days delinquent 11.00% 0.00%
Credit Enhancement 0.00% 0.00%
3 Month CPR 0.00% 0.00%
12 Month CPR 1.00% 1.00%
Maximum    
Investment Holdings [Line Items]    
Loss Severity 89.00% 85.00%
60 days delinquent 27.00% 40.00%
Credit Enhancement 50.00% 100.00%
3 Month CPR 24.00% 27.00%
12 Month CPR 19.00% 24.00%