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	Jun. 30, 2013	Dec. 31, 2012
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 27,437.6	$ 28,422.2
Non-Agency RMBS
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	66.4
Weighted Average Ultimate Default Rates	39.90%
Loss Severity Rates - Weighted Average	88.30%
Non-Agency RMBS | Lower Limit
Schedule of Available-for-sale Securities [Line Items]
Loss Severity Rates	41.90%
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	7.8
Weighted Average Ultimate Default Rates	18.90%
Loss Severity Rates - Weighted Average	57.80%
Non-Agency RMBS | Prime | Lower Limit
Schedule of Available-for-sale Securities [Line Items]
Loss Severity Rates	41.90%
Non-Agency RMBS | Prime | Upper Limit
Schedule of Available-for-sale Securities [Line Items]
Loss Severity Rates	65.30%
Non-Agency RMBS | Alt-A
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	12.6
Weighted Average Ultimate Default Rates	43.10%
Loss Severity Rates - Weighted Average	88.10%
Non-Agency RMBS | Alt-A | Lower Limit
Schedule of Available-for-sale Securities [Line Items]
Loss Severity Rates	88.10%
Non-Agency RMBS | Alt-A | Upper Limit
Schedule of Available-for-sale Securities [Line Items]
Loss Severity Rates	88.10%
Non-Agency RMBS | Subprime
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	32.0
Weighted Average Ultimate Default Rates	47.90%
Loss Severity Rates - Weighted Average	90.90%
Non-Agency RMBS | Subprime | Lower Limit
Schedule of Available-for-sale Securities [Line Items]
Loss Severity Rates	84.40%
Non-Agency RMBS | Subprime | Upper Limit
Schedule of Available-for-sale Securities [Line Items]
Loss Severity Rates	100.00%
Non-Agency RMBS | 2nd Lien
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 14.0
Weighted Average Ultimate Default Rates	33.10%
Loss Severity Rates - Weighted Average	99.50%
Non-Agency RMBS | 2nd Lien | Lower Limit
Schedule of Available-for-sale Securities [Line Items]
Loss Severity Rates	99.00%
Non-Agency RMBS | 2nd Lien | Upper Limit
Schedule of Available-for-sale Securities [Line Items]
Loss Severity Rates	100.00%