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	Dec. 31, 2014	Dec. 31, 2013
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 29,514.1	$ 28,382.3
Non-Agency RMBS
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	6.9
Weighted Average Ultimate Default Rates	25.30%
Loss Severity Rates - Weighted Average	81.10%
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	1.9
Weighted Average Ultimate Default Rates	5.00%
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 | 2nd Lien
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 5.0
Weighted Average Ultimate Default Rates	32.20%
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	99.00%
Non-Agency RMBS | 2nd Lien | Upper Limit
Schedule of Available-for-sale Securities [Line Items]
Loss Severity Rates	99.00%