Expected Loss to be Paid - Liquidation Rates and Key Assumptions in Base Case Expected Loss First Lien RMBS (Details)	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
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%	25.00%	25.00%
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%	25.00%	25.00%
Financing Receivable, Delinquent/Modified in Previous 12 Months [Member] | Subprime [Member]
Schedule of Expected Losses to be Paid [Line Items]
Liquidation Rate	20.00%	25.00%	25.00%
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%	35.00%	35.00%
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%	40.00%
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%	45.00%
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	40.00%	45.00%	45.00%
Financing Receivables, 60 to 89 Days Past Due [Member] | Option ARM [Member]
Schedule of Expected Losses to be Paid [Line Items]
Liquidation Rate	50.00%	50.00%	50.00%
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%	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	55.00%	55.00%	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	60.00%	55.00%	60.00%
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%	60.00%
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%
Financing Receivables, Bankruptcy [Member] | Option ARM [Member]
Schedule of Expected Losses to be Paid [Line Items]
Liquidation Rate	50.00%	50.00%	50.00%
Financing Receivables, Bankruptcy [Member] | Subprime [Member]
Schedule of Expected Losses to be Paid [Line Items]
Liquidation Rate	40.00%	40.00%	40.00%
Financing Receivable, Foreclosure [Member] | Alt-A and Prime [Member]
Schedule of Expected Losses to be Paid [Line Items]
Liquidation Rate	65.00%	65.00%	65.00%
Financing Receivable, Foreclosure [Member] | Option ARM [Member]
Schedule of Expected Losses to be Paid [Line Items]
Liquidation Rate	70.00%	65.00%	70.00%
Financing Receivable, Foreclosure [Member] | Subprime [Member]
Schedule of Expected Losses to be Paid [Line Items]
Liquidation Rate	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%
United States [Member] | Alt-A [Member] | 2005 and prior [Member] | Residential Mortgage-Backed Securities (RMBS) [Member]
Schedule of Expected Losses to be Paid [Line Items]
Initial loss severity	60.00%	60.00%	60.00%
United States [Member] | Alt-A [Member] | 2006 [Member] | Residential Mortgage-Backed Securities (RMBS) [Member]
Schedule of Expected Losses to be Paid [Line Items]
Initial loss severity	80.00%	80.00%	70.00%
United States [Member] | Alt-A [Member] | 2007 [Member] | Residential Mortgage-Backed Securities (RMBS) [Member]
Schedule of Expected Losses to be Paid [Line Items]
Initial loss severity	70.00%	70.00%	65.00%
United States [Member] | Alt-A [Member] | Minimum [Member] | Residential Mortgage-Backed Securities (RMBS) [Member]
Schedule of Expected Losses to be Paid [Line Items]
Plateau CDR	1.30%	1.00%	1.70%
Final CDR	0.10%	0.00%	0.10%
United States [Member] | Alt-A [Member] | Maximum [Member] | Residential Mortgage-Backed Securities (RMBS) [Member]
Schedule of Expected Losses to be Paid [Line Items]
Plateau CDR	9.80%	13.50%	26.40%
Final CDR	0.50%	0.70%	1.30%
United States [Member] | Alt-A [Member] | Weighted Average [Member] | Residential Mortgage-Backed Securities (RMBS) [Member]
Schedule of Expected Losses to be Paid [Line Items]
Plateau CDR	5.20%	5.70%	6.40%
Final CDR	0.30%	0.30%	0.30%
United States [Member] | Option ARM [Member] | 2005 and prior [Member] | Residential Mortgage-Backed Securities (RMBS) [Member]
Schedule of Expected Losses to be Paid [Line Items]
Initial loss severity	60.00%	60.00%	60.00%
United States [Member] | Option ARM [Member] | 2006 [Member] | Residential Mortgage-Backed Securities (RMBS) [Member]
Schedule of Expected Losses to be Paid [Line Items]
Initial loss severity	70.00%	70.00%	70.00%
United States [Member] | Option ARM [Member] | 2007 [Member] | Residential Mortgage-Backed Securities (RMBS) [Member]
Schedule of Expected Losses to be Paid [Line Items]
Initial loss severity	75.00%	75.00%	65.00%
United States [Member] | Option ARM [Member] | Minimum [Member] | Residential Mortgage-Backed Securities (RMBS) [Member]
Schedule of Expected Losses to be Paid [Line Items]
Plateau CDR	2.50%	3.20%	3.50%
Final CDR	0.10%	0.20%	0.20%
United States [Member] | Option ARM [Member] | Maximum [Member] | Residential Mortgage-Backed Securities (RMBS) [Member]
Schedule of Expected Losses to be Paid [Line Items]
Plateau CDR	7.00%	7.00%	10.30%
Final CDR	0.30%	0.30%	0.50%
United States [Member] | Option ARM [Member] | Weighted Average [Member] | Residential Mortgage-Backed Securities (RMBS) [Member]
Schedule of Expected Losses to be Paid [Line Items]
Plateau CDR	5.90%	5.60%	7.80%
Final CDR	0.30%	0.30%	0.40%
United States [Member] | Subprime [Member] | 2005 and prior [Member] | Residential Mortgage-Backed Securities (RMBS) [Member]
Schedule of Expected Losses to be Paid [Line Items]
Initial loss severity	80.00%	80.00%	75.00%
United States [Member] | Subprime [Member] | 2006 [Member] | Residential Mortgage-Backed Securities (RMBS) [Member]
Schedule of Expected Losses to be Paid [Line Items]
Initial loss severity	90.00%	90.00%	90.00%
United States [Member] | Subprime [Member] | 2007 [Member] | Residential Mortgage-Backed Securities (RMBS) [Member]
Schedule of Expected Losses to be Paid [Line Items]
Initial loss severity	95.00%	90.00%	90.00%
United States [Member] | Subprime [Member] | Minimum [Member] | Residential Mortgage-Backed Securities (RMBS) [Member]
Schedule of Expected Losses to be Paid [Line Items]
Plateau CDR	3.50%	2.80%	4.70%
Final CDR	0.20%	0.10%	0.20%
United States [Member] | Subprime [Member] | Maximum [Member] | Residential Mortgage-Backed Securities (RMBS) [Member]
Schedule of Expected Losses to be Paid [Line Items]
Plateau CDR	13.10%	14.10%	13.20%
Final CDR	0.70%	0.70%	0.70%
United States [Member] | Subprime [Member] | Weighted Average [Member] | Residential Mortgage-Backed Securities (RMBS) [Member]
Schedule of Expected Losses to be Paid [Line Items]
Plateau CDR	7.80%	8.10%	9.50%
Final CDR	0.40%	0.40%	0.40%