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	Mar. 31, 2012	Dec. 31, 2011
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 30,775.8	$ 30,092.6
Non-Agency RMBS
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	161.4
Weighted Average Ultimate Default Rates	41.60%
Loss Severity Rates - Weighted Average	73.70%
Non-Agency RMBS | Lower Limit
Schedule of Available-for-sale Securities [Line Items]
Loss Severity Rates	37.70%
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	23.6
Weighted Average Ultimate Default Rates	16.50%
Loss Severity Rates - Weighted Average	50.40%
Non-Agency RMBS | Prime | Lower Limit
Schedule of Available-for-sale Securities [Line Items]
Loss Severity Rates	37.70%
Non-Agency RMBS | Prime | Upper Limit
Schedule of Available-for-sale Securities [Line Items]
Loss Severity Rates	66.00%
Non-Agency RMBS | Alt-A
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	16.0
Weighted Average Ultimate Default Rates	42.80%
Loss Severity Rates - Weighted Average	69.40%
Non-Agency RMBS | Alt-A | Lower Limit
Schedule of Available-for-sale Securities [Line Items]
Loss Severity Rates	69.30%
Non-Agency RMBS | Alt-A | Upper Limit
Schedule of Available-for-sale Securities [Line Items]
Loss Severity Rates	69.40%
Non-Agency RMBS | Subprime
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	91.0
Weighted Average Ultimate Default Rates	51.00%
Loss Severity Rates - Weighted Average	71.80%
Non-Agency RMBS | Subprime | Lower Limit
Schedule of Available-for-sale Securities [Line Items]
Loss Severity Rates	65.70%
Non-Agency RMBS | Subprime | Upper Limit
Schedule of Available-for-sale Securities [Line Items]
Loss Severity Rates	84.90%
Non-Agency RMBS | 2nd Lien
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 30.8
Weighted Average Ultimate Default Rates	33.70%
Loss Severity Rates - Weighted Average	99.40%
Non-Agency RMBS | 2nd Lien | Lower Limit
Schedule of Available-for-sale Securities [Line Items]
Loss Severity Rates	98.70%
Non-Agency RMBS | 2nd Lien | Upper Limit
Schedule of Available-for-sale Securities [Line Items]
Loss Severity Rates	100.00%