Valuation Assumption Ranges for Level 3 Available-for-Sale Investment Securities (Detail)	Sep. 30, 2012
Minimum | Residential Prime Non-Agency Mortgage-Backed Securities
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Estimated lifetime prepayment rates	6.00%	[1]
Lifetime loss severity rates	25.00%	[1]
Discount margin	3.00%	[1]
Minimum | Residential Non-Prime Non-Agency Mortgage-Backed Securities
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Estimated lifetime prepayment rates	2.00%	[2]
Lifetime probability of default rates	3.00%	[2]
Lifetime loss severity rates	20.00%	[2]
Discount margin	3.00%	[2]
Minimum | Other asset-backed securities
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 | Residential Prime Non-Agency Mortgage-Backed Securities
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Estimated lifetime prepayment rates	20.00%	[1]
Lifetime probability of default rates	5.00%	[1]
Lifetime loss severity rates	80.00%	[1]
Discount margin	7.00%	[1]
Maximum | Residential Non-Prime Non-Agency Mortgage-Backed Securities
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Estimated lifetime prepayment rates	10.00%	[2]
Lifetime probability of default rates	10.00%	[2]
Lifetime loss severity rates	70.00%	[2]
Discount margin	9.00%	[2]
Maximum | Other asset-backed securities
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 | Residential Prime Non-Agency Mortgage-Backed Securities
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Estimated lifetime prepayment rates	13.00%	[1]
Lifetime probability of default rates	3.00%	[1]
Lifetime loss severity rates	42.00%	[1]
Discount margin	5.00%	[1]
Average | Residential Non-Prime Non-Agency Mortgage-Backed Securities
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Estimated lifetime prepayment rates	6.00%	[2]
Lifetime probability of default rates	6.00%	[2]
Lifetime loss severity rates	53.00%	[2]
Discount margin	6.00%	[2]
Average | Other asset-backed securities
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%

[1] Prime securities are those designated as such by the issuer at origination. When an issuer designation is unavailable, the Company determines at acquisition date the categorization based on asset pool characteristics (such as weighted-average credit score, loan-to-value, loan type, prevalence of low documentation loans) and deal performance (such as pool delinquencies and security market spreads).

[2] Includes all securities not meeting the conditions to be designated as prime.