Significant Inputs Considered in Determining the Measurement of Credit Losses Recognized in Earnings for Private Residential MBS (Detail)	3 Months Ended
	Mar. 31, 2012	Mar. 31, 2011
Lower Limit
Schedule of Available-for-sale Securities [Line Items]
Current default rate	2.00%	5.00%
Prepayment rate	8.00%	13.00%
Inputs Considered In Determining Measurement Of Credit Losses Loss Severity	47.00%	39.00%
Upper Limit
Schedule of Available-for-sale Securities [Line Items]
Current default rate	6.00%	7.00%
Prepayment rate	16.00%	19.00%
Inputs Considered In Determining Measurement Of Credit Losses Loss Severity	52.00%	43.00%