Fair Values of Assets and Liabilities - Valuation Assumption Ranges for Level 3 Available - for - Sale Investment Securities (Detail)	Mar. 31, 2013
Minimum [Member] | Residential Prime Non-Agency Mortgage-Backed Securities [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Estimated lifetime prepayment rates	6.00%
Lifetime loss severity rates	25.00%
Discount margin	2.00%
Minimum [Member] | Residential Non-Prime Non-Agency Mortgage-Backed Securities [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Estimated lifetime prepayment rates	2.00%
Lifetime probability of default rates	4.00%
Lifetime loss severity rates	15.00%
Discount margin	2.00%
Minimum [Member] | Asset Backed Securities Other [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Estimated lifetime prepayment rates	6.00%
Lifetime probability of default rates	4.00%
Lifetime loss severity rates	40.00%
Discount margin	18.00%
Maximum [Member] | Residential Prime Non-Agency Mortgage-Backed Securities [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	70.00%
Discount margin	7.00%
Maximum [Member] | Residential Non-Prime Non-Agency Mortgage-Backed Securities [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Estimated lifetime prepayment rates	10.00%
Lifetime probability of default rates	10.00%
Lifetime loss severity rates	70.00%
Discount margin	6.00%
Maximum [Member] | Asset Backed Securities Other [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Estimated lifetime prepayment rates	6.00%
Lifetime probability of default rates	4.00%
Lifetime loss severity rates	40.00%
Discount margin	18.00%
Average [Member] | Residential Prime Non-Agency Mortgage-Backed Securities [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Estimated lifetime prepayment rates	13.00%
Lifetime probability of default rates	3.00%
Lifetime loss severity rates	43.00%
Discount margin	4.00%
Average [Member] | Residential Non-Prime Non-Agency Mortgage-Backed Securities [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Estimated lifetime prepayment rates	6.00%
Lifetime probability of default rates	7.00%
Lifetime loss severity rates	55.00%
Discount margin	3.00%
Average [Member] | Asset Backed Securities Other [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Estimated lifetime prepayment rates	6.00%
Lifetime probability of default rates	4.00%
Lifetime loss severity rates	40.00%
Discount margin	18.00%