Book Values, Ultimate Default Rates, and Loss Severity Rates for Non-Agency Residential Mortgage-Backed Securities Portfolio, by Security Type (Detail) (USD $) In Millions, unless otherwise specified	Sep. 30, 2012	Dec. 31, 2011
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 27,896.8	$ 30,092.6
Non-Agency RMBS
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	127.5
Weighted Average Ultimate Default Rates	40.00%
Loss Severity Rates - Weighted Average	73.50%
Non-Agency RMBS | Lower Limit
Schedule of Available-for-sale Securities [Line Items]
Loss Severity Rates	35.90%
Non-Agency RMBS | Upper Limit
Schedule of Available-for-sale Securities [Line Items]
Loss Severity Rates	100.00%
Non-Agency RMBS | Prime
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	22.3
Weighted Average Ultimate Default Rates	15.00%
Loss Severity Rates - Weighted Average	46.20%
Non-Agency RMBS | Prime | Lower Limit
Schedule of Available-for-sale Securities [Line Items]
Loss Severity Rates	35.90%
Non-Agency RMBS | Prime | Upper Limit
Schedule of Available-for-sale Securities [Line Items]
Loss Severity Rates	57.80%
Non-Agency RMBS | Alt-A
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	13.7
Weighted Average Ultimate Default Rates	44.00%
Loss Severity Rates - Weighted Average	68.90%
Non-Agency RMBS | Alt-A | Lower Limit
Schedule of Available-for-sale Securities [Line Items]
Loss Severity Rates	66.80%
Non-Agency RMBS | Alt-A | Upper Limit
Schedule of Available-for-sale Securities [Line Items]
Loss Severity Rates	68.90%
Non-Agency RMBS | Subprime
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	65.4
Weighted Average Ultimate Default Rates	50.80%
Loss Severity Rates - Weighted Average	73.50%
Non-Agency RMBS | Subprime | Lower Limit
Schedule of Available-for-sale Securities [Line Items]
Loss Severity Rates	63.60%
Non-Agency RMBS | Subprime | Upper Limit
Schedule of Available-for-sale Securities [Line Items]
Loss Severity Rates	83.80%
Non-Agency RMBS | 2nd Lien
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 26.1
Weighted Average Ultimate Default Rates	33.50%
Loss Severity Rates - Weighted Average	99.20%
Non-Agency RMBS | 2nd Lien | Lower Limit
Schedule of Available-for-sale Securities [Line Items]
Loss Severity Rates	98.60%
Non-Agency RMBS | 2nd Lien | Upper Limit
Schedule of Available-for-sale Securities [Line Items]
Loss Severity Rates	100.00%