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, 2014	Dec. 31, 2013
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 28,046.2	$ 28,382.3
Non-Agency RMBS
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	18.8
Weighted Average Ultimate Default Rates	33.10%
Loss Severity Rates - Weighted Average	70.30%
Non-Agency RMBS | Lower Limit
Schedule of Available-for-sale Securities [Line Items]
Loss Severity Rates	32.10%
Non-Agency RMBS | Upper Limit
Schedule of Available-for-sale Securities [Line Items]
Loss Severity Rates	99.00%
Non-Agency RMBS | Prime
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	2.0
Weighted Average Ultimate Default Rates	5.40%
Loss Severity Rates - Weighted Average	32.10%
Non-Agency RMBS | Prime | Lower Limit
Schedule of Available-for-sale Securities [Line Items]
Loss Severity Rates	32.10%
Non-Agency RMBS | Prime | Upper Limit
Schedule of Available-for-sale Securities [Line Items]
Loss Severity Rates	32.10%
Non-Agency RMBS | Alt-A
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	11.2
Weighted Average Ultimate Default Rates	40.00%
Loss Severity Rates - Weighted Average	62.80%
Non-Agency RMBS | Alt-A | Lower Limit
Schedule of Available-for-sale Securities [Line Items]
Loss Severity Rates	62.80%
Non-Agency RMBS | Alt-A | Upper Limit
Schedule of Available-for-sale Securities [Line Items]
Loss Severity Rates	62.80%
Non-Agency RMBS | 2nd Lien
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 5.6
Weighted Average Ultimate Default Rates	32.70%
Loss Severity Rates - Weighted Average	99.00%
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	99.00%