Investment Securities - Summary of Ranges Used in Valuation of Non-Agency Mortgage-Backed Securities Other Than Temporarily Impaired (Detail)	3 Months Ended
	Jun. 30, 2014
Prime Minimum [Member]
Principal Valuation Assumptions Debt Securities Other Than Temporarily Impaired [Line Items]
Estimated lifetime prepayment rates	15.00%
Lifetime probability of default rates	4.00%
Lifetime loss severity rates	15.00%
Prime Maximum [Member]
Principal Valuation Assumptions Debt Securities Other Than Temporarily Impaired [Line Items]
Estimated lifetime prepayment rates	20.00%
Lifetime probability of default rates	7.00%
Lifetime loss severity rates	55.00%
Prime Average [Member]
Principal Valuation Assumptions Debt Securities Other Than Temporarily Impaired [Line Items]
Estimated lifetime prepayment rates	16.00%
Lifetime probability of default rates	6.00%
Lifetime loss severity rates	40.00%
Non-Prime Minimum [Member]
Principal Valuation Assumptions Debt Securities Other Than Temporarily Impaired [Line Items]
Estimated lifetime prepayment rates	1.00%
Lifetime probability of default rates	8.00%
Lifetime loss severity rates	40.00%
Non-Prime Maximum [Member]
Principal Valuation Assumptions Debt Securities Other Than Temporarily Impaired [Line Items]
Estimated lifetime prepayment rates	8.00%
Lifetime probability of default rates	10.00%
Lifetime loss severity rates	75.00%
Non-Prime Average [Member]
Principal Valuation Assumptions Debt Securities Other Than Temporarily Impaired [Line Items]
Estimated lifetime prepayment rates	3.00%
Lifetime probability of default rates	9.00%
Lifetime loss severity rates	63.00%