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Mortgage-Backed Securities - Investment Holdings - Narrative (Detail)
$ in Millions
12 Months Ended
Dec. 31, 2016
USD ($)
point
Dec. 31, 2015
USD ($)
Dec. 31, 2014
USD ($)
Interest-Only RMBS      
Investment Holdings [Line Items]      
Interest-only RMBS, net unrealized gain (loss) $ (48) $ (45)  
Interest-only RMBS, amortized cost 458 564  
Interest-only RMBS, fair value 410 519  
Agency RMBS - Pass-through      
Investment Holdings [Line Items]      
Gross unrealized loss 100 60  
Non-agency MBS      
Investment Holdings [Line Items]      
Gross unrealized loss $ 1 $ 16  
Non-Agency RMBS Deemed To Be Equivalent To Alt-A Quality      
Investment Holdings [Line Items]      
Percentage in Alt-A collateral 68.00% 64.00%  
Non Agency Residential MBS Deemed To Be Equivalent To Prime Quality      
Investment Holdings [Line Items]      
Percentage in prime collateral 14.00% 17.00%  
Re-REMIC Security      
Investment Holdings [Line Items]      
Fair value of securities exchanged $ 65 $ 95 $ 89
Gain (loss) on sale of investments $ 13 $ 24 $ 23
Minimum | Alt-A      
Investment Holdings [Line Items]      
FICO Score | point 680    
Maximum | Alt-A      
Investment Holdings [Line Items]      
FICO Score | point 720    
Alt-A securities underlying collateral of full documented loans 30.00%