Investment Securities - Summary of Ranges Used in Valuation of Non-Agency Mortgage-Backed Securities Other Than Temporarily Impaired (Detail)	Mar. 31, 2013	Dec. 31, 2012
Prime Minimum [Member]
Principal Valuation Assumptions Debt Securities Other Than Temporarily Impaired [Line Items]
Estimated lifetime prepayment rates	8.00%	6.00%
Lifetime probability of default rates	4.00%	3.00%
Lifetime loss severity rates	45.00%	40.00%
Prime Maximum [Member]
Principal Valuation Assumptions Debt Securities Other Than Temporarily Impaired [Line Items]
Estimated lifetime prepayment rates	18.00%	22.00%
Lifetime probability of default rates	6.00%	6.00%
Lifetime loss severity rates	50.00%	50.00%
Prime Average [Member]
Principal Valuation Assumptions Debt Securities Other Than Temporarily Impaired [Line Items]
Estimated lifetime prepayment rates	16.00%	14.00%
Lifetime probability of default rates	4.00%	4.00%
Lifetime loss severity rates	48.00%	47.00%
Non-Prime Minimum [Member]
Principal Valuation Assumptions Debt Securities Other Than Temporarily Impaired [Line Items]
Estimated lifetime prepayment rates	2.00%	3.00%
Lifetime probability of default rates	4.00%	3.00%
Lifetime loss severity rates	50.00%	45.00%
Non-Prime Maximum [Member]
Principal Valuation Assumptions Debt Securities Other Than Temporarily Impaired [Line Items]
Estimated lifetime prepayment rates	10.00%	10.00%
Lifetime probability of default rates	10.00%	10.00%
Lifetime loss severity rates	70.00%	65.00%
Non-Prime Average [Member]
Principal Valuation Assumptions Debt Securities Other Than Temporarily Impaired [Line Items]
Estimated lifetime prepayment rates	6.00%	6.00%
Lifetime probability of default rates	7.00%	7.00%
Lifetime loss severity rates	60.00%	56.00%