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Fair Values of Assets and Liabilities - Valuation Assumption Ranges for Level 3 Available -for-Sale Investment Securities (Detail) - Level 3 [Member]
3 Months Ended
Mar. 31, 2016
Minimum [Member] | Non-Agency Prime [Member] | Mortgage-Backed Securities Residential [Member]  
Fair Value Inputs, Assets, Quantitative Information [Line Items]  
Estimated lifetime prepayment rates 5.00%
Lifetime loss severity rates 15.00%
Discount margin 2.00%
Minimum [Member] | Non-Agency Non-Prime [Member] | Mortgage-Backed Securities Residential [Member]  
Fair Value Inputs, Assets, Quantitative Information [Line Items]  
Estimated lifetime prepayment rates 3.00%
Lifetime probability of default rates 3.00%
Lifetime loss severity rates 20.00%
Discount margin 1.00%
Minimum [Member] | Asset-Backed Securities Other [Member] | Asset-Backed Securities [Member]  
Fair Value Inputs, Assets, Quantitative Information [Line Items]  
Estimated lifetime prepayment rates 6.00%
Lifetime probability of default rates 5.00%
Lifetime loss severity rates 40.00%
Discount margin 6.00%
Maximum [Member] | Non-Agency Prime [Member] | Mortgage-Backed Securities Residential [Member]  
Fair Value Inputs, Assets, Quantitative Information [Line Items]  
Estimated lifetime prepayment rates 20.00%
Lifetime probability of default rates 6.00%
Lifetime loss severity rates 60.00%
Discount margin 6.00%
Maximum [Member] | Non-Agency Non-Prime [Member] | Mortgage-Backed Securities Residential [Member]  
Fair Value Inputs, Assets, Quantitative Information [Line Items]  
Estimated lifetime prepayment rates 17.00%
Lifetime probability of default rates 12.00%
Lifetime loss severity rates 70.00%
Discount margin 11.00%
Maximum [Member] | Asset-Backed Securities Other [Member] | Asset-Backed Securities [Member]  
Fair Value Inputs, Assets, Quantitative Information [Line Items]  
Estimated lifetime prepayment rates 6.00%
Lifetime probability of default rates 5.00%
Lifetime loss severity rates 40.00%
Discount margin 6.00%
Average [Member] | Non-Agency Prime [Member] | Mortgage-Backed Securities Residential [Member]  
Fair Value Inputs, Assets, Quantitative Information [Line Items]  
Estimated lifetime prepayment rates 14.00%
Lifetime probability of default rates 4.00%
Lifetime loss severity rates 32.00%
Discount margin 3.00%
Average [Member] | Non-Agency Non-Prime [Member] | Mortgage-Backed Securities Residential [Member]  
Fair Value Inputs, Assets, Quantitative Information [Line Items]  
Estimated lifetime prepayment rates 8.00%
Lifetime probability of default rates 7.00%
Lifetime loss severity rates 51.00%
Discount margin 3.00%
Average [Member] | Asset-Backed Securities Other [Member] | Asset-Backed Securities [Member]  
Fair Value Inputs, Assets, Quantitative Information [Line Items]  
Estimated lifetime prepayment rates 6.00%
Lifetime probability of default rates 5.00%
Lifetime loss severity rates 40.00%
Discount margin 6.00%