Summary of Significant Valuation Assumptions for Available for Sale Securities (Detail)	Sep. 30, 2012
Residential Prime | Minimum
Schedule of Available-for-sale Securities [Line Items]
Prepayment rates	4.00%
Loss severity	26.00%
Projected losses	0.00%
Residential Prime | Maximum
Schedule of Available-for-sale Securities [Line Items]
Prepayment rates	19.00%
Loss severity	59.00%
Projected losses	25.00%
Residential non-prime | Minimum
Schedule of Available-for-sale Securities [Line Items]
Prepayment rates	3.00%
Loss severity	29.00%
Projected losses	6.00%
Residential non-prime | Maximum
Schedule of Available-for-sale Securities [Line Items]
Prepayment rates	10.00%
Loss severity	60.00%
Projected losses	43.00%
Commercial | Minimum
Schedule of Available-for-sale Securities [Line Items]
Loss severity	33.00%
Projected losses	2.00%
Commercial | Maximum
Schedule of Available-for-sale Securities [Line Items]
Loss severity	50.00%
Projected losses	9.00%