Expected Losses to be Paid - Key Assumptions in Base Case Expected Loss Estimates Second Lien RMBS (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended	12 Months Ended													3 Months Ended					12 Months Ended										3 Months Ended		12 Months Ended																						3 Months Ended	12 Months Ended												3 Months Ended
	Mar. 31, 2014	Dec. 31, 2013	Mar. 31, 2014 Alt-A first lien		Dec. 31, 2013 Alt-A first lien		Mar. 31, 2014 Alt-A first lien Minimum		Dec. 31, 2013 Alt-A first lien Minimum		Mar. 31, 2014 Alt-A first lien Maximum		Dec. 31, 2013 Alt-A first lien Maximum		Mar. 31, 2014 Second lien Curve	Mar. 31, 2014 Second lien Minimum	Mar. 31, 2014 Second lien Maximum	Mar. 31, 2014 Close ended second lien		Dec. 31, 2013 Close ended second lien		Mar. 31, 2014 Close ended second lien Minimum		Dec. 31, 2013 Close ended second lien Minimum		Mar. 31, 2014 Close ended second lien Maximum		Dec. 31, 2013 Close ended second lien Maximum		Mar. 31, 2014 HELOCs		Dec. 31, 2013 HELOCs		Mar. 31, 2014 HELOCs Minimum		Dec. 31, 2013 HELOCs Minimum		Mar. 31, 2014 HELOCs Maximum		Dec. 31, 2013 HELOCs Maximum		Mar. 31, 2014 Option ARM		Dec. 31, 2013 Option ARM		Mar. 31, 2014 Option ARM Minimum		Dec. 31, 2013 Option ARM Minimum		Mar. 31, 2014 Option ARM Maximum		Dec. 31, 2013 Option ARM Maximum		Dec. 31, 2012 Subprime	Dec. 31, 2013 Subprime		Mar. 31, 2014 Subprime		Mar. 31, 2014 Subprime Minimum		Dec. 31, 2013 Subprime Minimum		Mar. 31, 2014 Subprime Maximum		Dec. 31, 2013 Subprime Maximum		Mar. 31, 2014 Base Scenario Second lien	Mar. 31, 2014 Scenario 1 Second lien	Mar. 31, 2014 Scenario 1 Close ended second lien	Mar. 31, 2014 Scenario 1 HELOCs	Mar. 31, 2014 Scenario 2 Second lien	Mar. 31, 2014 Scenario 2 Close ended second lien	Mar. 31, 2014 Scenario 2 HELOCs
Guarantor Obligations [Line Items]
Period from initial to final conditional prepayment rate (in months)		2 years 6 months
Period until which loss severity rate would continue (in months)		18 months																																																				9 months	9 months
Plateau conditional default rate (as a percent)							2.30%	[1]	2.80%	[1]	18.40%	[1]	18.40%	[1]								6.70%	[1]	7.30%	[1]	15.50%	[1]	15.10%	[1]					1.90%	[1]	2.30%	[1]	7.30%	[1]	7.70%	[1]					3.80%	[1]	4.90%	[1]	16.80%	[1]	16.80%	[1]						5.90%	[1]	5.60%	[1]	16.30%	[1]	16.20%	[1]
Intermediate Conditional Default Rate							0.50%	[1]	0.60%	[1]	3.70%	[1]	3.70%	[1]																																0.80%	[1]	1.00%	[1]	3.40%	[1]	3.40%	[1]						1.20%	[1]	1.10%	[1]	3.30%	[1]	3.20%	[1]
Final conditional default rate trended down to (as a percent)							0.10%	[1]	0.10%	[1]	0.90%	[1]	0.90%	[1]								3.50%	[1]	3.50%	[1]	9.10%	[1]	9.10%	[1]					0.40%	[1]	0.40%	[1]	3.20%	[1]	3.20%	[1]					0.20%	[1]	0.20%	[1]	0.80%	[1]	0.80%	[1]						0.30%	[1]	0.30%	[1]	0.80%	[1]	0.80%	[1]
Loss severity (as a percent)			65.00%	[1]	65.00%	[1]												98.00%	[1]	98.00%	[1]									98.00%	[1]	98.00%	[1]									65.00%	[1]	65.00%	[1]										90.00%	[1]	90.00%	[1]
Initial conditional prepayment rate (as a percent)							0.90%	[1]	0.00%	[1]	33.90%	[1]	34.20%	[1]								2.90%	[1]	3.10%	[1]	12.80%	[1]	12.00%	[1]					2.30%	[1]	2.70%	[1]	21.00%	[1]	21.50%	[1]					0.80%	[1]	0.40%	[1]	12.20%	[1]	13.10%	[1]						0.00%	[1]	0.00%	[1]	11.60%	[1]	15.70%	[1]
Final conditional prepayment rate (as a percent)			15.00%	[1]	15.00%	[1]									10.00%			10.00%	[1]	10.00%	[1]									10.00%	[1]	10.00%	[1]									15.00%	[1]	15.00%	[1]										15.00%	[1]	15.00%	[1]
Expected period until final conditional default rate (in months)																		34 months	[1]	34 months										34 months		34 months	[1]
Typical past due period for loans to be charged off (in days)															180 days
First delinquency category (in days)																30 days	59 days
Second delinquency category (in days)																60 days	89 days
Third delinquency category (in days)																90 days	119 days
Fourth delinquency category (in days)																120 days	149 days
Fifth delinquency category (in days)																150 days	179 days
Period of Default Estimate															5 months
Number of Preceding Months Liquidation Rates Used to Estimate Loan Default Rate															12 months
Number of Months the Conditional Default Rate is Calculated Applying Liquidation Rates to Current Period Past Due Balances Excluding Thirty to Fifty Nine Days Past Due															4 months
Period of decrease in conditional default rate until final rate is achieved (in months)															28 months																																																				28 months	33 months
Stress period (in months)															34 months																																																				34 months	42 months			24 months
Number of Monthly Delinquency Categories															5 months
Period of constant conditional default rate (in months)															1 month																																																				1 month				1 month
Loss Recovery Assumption															2.00%
Conditional Prepayment Rate Base Case Average Number of Months															3 months
Number of conditional default rate curves modeled in estimating losses															3
Increased plateau period used to calculate potential change in loss estimate (in months)																																																																				4 months
Loss Estimate Sensitivity Analysis Increase in Conditional Default Rate Ramp Down Period	5 months
Loss Estimate Sensitivity Analysis Change in Estimate for Increased Conditional Default Rate Plateau Period																																																																					$ 2	$ 24
Decreased ramp down period used to calculate potential change in loss estimate (in months)																																																																							18 months
Loss Estimate Sensitivity Analysis Change in Estimate for Decreased Conditional Default Rate Ramp Down Period																																																																								$ 2	$ 22

[1] Represents variables for most heavily weighted scenario (the “base case”).