Expected Loss to be Paid - Liquidation Rates and Key Assumptions in Base Case Expected Loss First Lien RMBS (Details) - scenario	3 Months Ended	6 Months Ended	9 Months Ended	12 Months Ended
	Sep. 30, 2018	Jun. 30, 2018	Sep. 30, 2018	Dec. 31, 2017
Financing Receivable, Delinquent/Modified in Previous 12 Months [Member] | Subprime [Member]
Schedule of Expected Losses to be Paid [Line Items]
Liquidation Rate	20.00%	20.00%	20.00%	20.00%
Financing Receivable, Delinquent/Modified in Previous 12 Months [Member] | Alt-A and Prime [Member]
Schedule of Expected Losses to be Paid [Line Items]
Liquidation Rate	20.00%	20.00%	20.00%	20.00%
Financing Receivable, Delinquent/Modified in Previous 12 Months [Member] | Option ARM [Member]
Schedule of Expected Losses to be Paid [Line Items]
Liquidation Rate	20.00%	20.00%	20.00%	20.00%
Financing Receivables, 30 to 59 Days Past Due [Member] | Subprime [Member]
Schedule of Expected Losses to be Paid [Line Items]
Liquidation Rate	40.00%	40.00%	40.00%	40.00%
Financing Receivables, 30 to 59 Days Past Due [Member] | Alt-A and Prime [Member]
Schedule of Expected Losses to be Paid [Line Items]
Liquidation Rate	30.00%	30.00%	30.00%	30.00%
Financing Receivables, 30 to 59 Days Past Due [Member] | Option ARM [Member]
Schedule of Expected Losses to be Paid [Line Items]
Liquidation Rate	35.00%	35.00%	35.00%	35.00%
Financing Receivables, 60 to 89 Days Past Due [Member] | Subprime [Member]
Schedule of Expected Losses to be Paid [Line Items]
Liquidation Rate	50.00%	50.00%	50.00%	50.00%
Financing Receivables, 60 to 89 Days Past Due [Member] | Alt-A and Prime [Member]
Schedule of Expected Losses to be Paid [Line Items]
Liquidation Rate	35.00%	35.00%	35.00%	40.00%
Financing Receivables, 60 to 89 Days Past Due [Member] | Option ARM [Member]
Schedule of Expected Losses to be Paid [Line Items]
Liquidation Rate	45.00%	45.00%	45.00%	50.00%
Financing Receivables, Equal to Greater than 90 Days Past Due [Member] | Subprime [Member]
Schedule of Expected Losses to be Paid [Line Items]
Liquidation Rate	55.00%	55.00%	55.00%	55.00%
Financing Receivables, Equal to Greater than 90 Days Past Due [Member] | Alt-A and Prime [Member]
Schedule of Expected Losses to be Paid [Line Items]
Liquidation Rate	40.00%	40.00%	40.00%	55.00%
Financing Receivables, Equal to Greater than 90 Days Past Due [Member] | Option ARM [Member]
Schedule of Expected Losses to be Paid [Line Items]
Liquidation Rate	55.00%	55.00%	55.00%	60.00%
Financing Receivables, Bankruptcy [Member] | Subprime [Member]
Schedule of Expected Losses to be Paid [Line Items]
Liquidation Rate	40.00%	40.00%	40.00%	40.00%
Financing Receivables, Bankruptcy [Member] | Alt-A and Prime [Member]
Schedule of Expected Losses to be Paid [Line Items]
Liquidation Rate	45.00%	45.00%	45.00%	45.00%
Financing Receivables, Bankruptcy [Member] | Option ARM [Member]
Schedule of Expected Losses to be Paid [Line Items]
Liquidation Rate	50.00%	50.00%	50.00%	50.00%
Financing Receivable, Foreclosure [Member] | Subprime [Member]
Schedule of Expected Losses to be Paid [Line Items]
Liquidation Rate	65.00%	65.00%	65.00%	65.00%
Financing Receivable, Foreclosure [Member] | Alt-A and Prime [Member]
Schedule of Expected Losses to be Paid [Line Items]
Liquidation Rate	55.00%	55.00%	55.00%	65.00%
Financing Receivable, Foreclosure [Member] | Option ARM [Member]
Schedule of Expected Losses to be Paid [Line Items]
Liquidation Rate	65.00%	65.00%	65.00%	70.00%
Financing Receivable, Real Estate Owned [Member]
Schedule of Expected Losses to be Paid [Line Items]
Liquidation Rate	100.00%	100.00%	100.00%	100.00%
United States [Member] | Alt-A and Prime [Member] | 2005 and prior [Member] | RMBS [Member]
Schedule of Expected Losses to be Paid [Line Items]
Initial loss severity	60.00%	60.00%	60.00%	60.00%
United States [Member] | Alt-A and Prime [Member] | 2006 [Member] | RMBS [Member]
Schedule of Expected Losses to be Paid [Line Items]
Initial loss severity	70.00%	80.00%	70.00%	80.00%
United States [Member] | Alt-A and Prime [Member] | 2007 [Member] | RMBS [Member]
Schedule of Expected Losses to be Paid [Line Items]
Initial loss severity	70.00%	70.00%	70.00%	70.00%
United States [Member] | Alt-A and Prime [Member] | Minimum [Member] | RMBS [Member]
Schedule of Expected Losses to be Paid [Line Items]
Plateau CDR	0.90%	0.70%	0.90%	1.30%
Final CDR	0.00%	0.00%	0.00%	0.10%
United States [Member] | Alt-A and Prime [Member] | Maximum [Member] | RMBS [Member]
Schedule of Expected Losses to be Paid [Line Items]
Plateau CDR	11.20%	12.20%	11.20%	9.80%
Final CDR	0.60%	0.60%	0.60%	0.50%
United States [Member] | Alt-A and Prime [Member] | Weighted Average [Member] | RMBS [Member]
Schedule of Expected Losses to be Paid [Line Items]
Plateau CDR	4.60%	4.70%	4.60%	5.20%
Final CDR	0.20%	0.20%	0.20%	0.30%
United States [Member] | Option ARM [Member] | 2005 and prior [Member] | RMBS [Member]
Schedule of Expected Losses to be Paid [Line Items]
Initial loss severity	60.00%	60.00%	60.00%	60.00%
United States [Member] | Option ARM [Member] | 2006 [Member] | RMBS [Member]
Schedule of Expected Losses to be Paid [Line Items]
Initial loss severity	60.00%	70.00%	60.00%	70.00%
United States [Member] | Option ARM [Member] | 2007 [Member] | RMBS [Member]
Schedule of Expected Losses to be Paid [Line Items]
Initial loss severity	70.00%	75.00%	70.00%	75.00%
United States [Member] | Option ARM [Member] | Minimum [Member] | RMBS [Member]
Schedule of Expected Losses to be Paid [Line Items]
Plateau CDR	2.10%	2.10%	2.10%	2.50%
Final CDR	0.10%	0.10%	0.10%	0.10%
United States [Member] | Option ARM [Member] | Maximum [Member] | RMBS [Member]
Schedule of Expected Losses to be Paid [Line Items]
Plateau CDR	8.70%	8.50%	8.70%	7.00%
Final CDR	0.40%	0.40%	0.40%	0.30%
United States [Member] | Option ARM [Member] | Weighted Average [Member] | RMBS [Member]
Schedule of Expected Losses to be Paid [Line Items]
Plateau CDR	5.80%	5.90%	5.80%	5.90%
Final CDR	0.30%	0.30%	0.30%	0.30%
United States [Member] | Subprime [Member] | 2005 and prior [Member] | RMBS [Member]
Schedule of Expected Losses to be Paid [Line Items]
Initial loss severity	80.00%	80.00%	80.00%	80.00%
United States [Member] | Subprime [Member] | 2006 [Member] | RMBS [Member]
Schedule of Expected Losses to be Paid [Line Items]
Initial loss severity	75.00%	85.00%	75.00%	90.00%
United States [Member] | Subprime [Member] | 2007 [Member] | RMBS [Member]
Schedule of Expected Losses to be Paid [Line Items]
Initial loss severity	95.00%	95.00%	95.00%	95.00%
United States [Member] | Subprime [Member] | Minimum [Member] | RMBS [Member]
Schedule of Expected Losses to be Paid [Line Items]
Plateau CDR	2.30%	3.20%	2.30%	3.50%
Final CDR	0.10%	0.20%	0.10%	0.20%
United States [Member] | Subprime [Member] | Maximum [Member] | RMBS [Member]
Schedule of Expected Losses to be Paid [Line Items]
Plateau CDR	18.60%	18.40%	18.60%	13.10%
Final CDR	0.90%	0.90%	0.90%	0.70%
United States [Member] | Subprime [Member] | Weighted Average [Member] | RMBS [Member]
Schedule of Expected Losses to be Paid [Line Items]
Plateau CDR	6.70%	6.90%	6.70%	7.80%
Final CDR	0.30%	0.30%	0.30%	0.40%
First Lien [Member] | United States [Member] | RMBS [Member]
Schedule of Expected Losses to be Paid [Line Items]
Projected loss assumptions, Final CDR, Period for voluntary prepayments to continue			12 months
Number of scenarios weighted in estimating expected losses	5	5	5	5
Base Scenario [Member] | First Lien [Member] | United States [Member] | RMBS [Member]
Schedule of Expected Losses to be Paid [Line Items]
Projected loss assumptions, Final CDR, Period for voluntary prepayments to continue			4 years 9 months
Final CPR	15.00%	15.00%	15.00%	15.00%