Significant Inputs Considered in Determining the Measurement of Credit Losses Recognized in Earnings for Private Residential MBS (Detail)	9 Months Ended	12 Months Ended
	Sep. 30, 2011	Dec. 31, 2010
Lower Limit
Schedule of Available-for-sale Securities [Line Items]
Current default rate	4.00%	2.00%
Prepayment rate	12.00%	14.00%
Loss severity	39.00%	37.00%
Upper Limit
Schedule of Available-for-sale Securities [Line Items]
Current default rate	8.00%	7.00%
Prepayment rate	22.00%	22.00%
Loss severity	44.00%	46.00%