Expected Loss to be Paid - Key Assumptions in Base Case Expected Loss (Details) - Residential Mortgage-Backed Securities (RMBS) [Member] - United States [Member] - Home equity lines of credit (HELOCs) [Member]	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Schedule of Expected Losses to be Paid [Line Items]
Loss severity	98.00%	98.00%	98.00%
Minimum [Member]
Schedule of Expected Losses to be Paid [Line Items]
Plateau CDR	2.70%	3.50%	4.90%
Final CDR trended down to	2.50%	0.50%	0.50%
Maximum [Member]
Schedule of Expected Losses to be Paid [Line Items]
Plateau CDR	19.90%	24.80%	23.50%
Final CDR trended down to	3.20%	3.20%	3.20%
Weighted Average [Member]
Schedule of Expected Losses to be Paid [Line Items]
Plateau CDR	11.40%	13.60%	10.30%
Final CDR trended down to	2.50%	1.30%	1.20%
Financing Receivable, Modified in Previous 12 Months [Member]
Schedule of Expected Losses to be Paid [Line Items]
Insured Financial Obligations, Projected Loss Assumptions, Liquidation Rate	20.00%	25.00%	25.00%
Financing Receivables, 30 to 59 Days Past Due [Member]
Schedule of Expected Losses to be Paid [Line Items]
Insured Financial Obligations, Projected Loss Assumptions, Liquidation Rate	45.00%	50.00%	50.00%
Financing Receivables, 60 to 89 Days Past Due [Member]
Schedule of Expected Losses to be Paid [Line Items]
Insured Financial Obligations, Projected Loss Assumptions, Liquidation Rate	60.00%	65.00%	65.00%
Financing Receivables, Equal to Greater than 90 Days Past Due [Member]
Schedule of Expected Losses to be Paid [Line Items]
Insured Financial Obligations, Projected Loss Assumptions, Liquidation Rate	75.00%	80.00%	75.00%
Financing Receivables, Bankruptcy [Member]
Schedule of Expected Losses to be Paid [Line Items]
Insured Financial Obligations, Projected Loss Assumptions, Liquidation Rate	55.00%	55.00%	55.00%
Financing Receivable, Foreclosure [Member]
Schedule of Expected Losses to be Paid [Line Items]
Insured Financial Obligations, Projected Loss Assumptions, Liquidation Rate	70.00%	75.00%	75.00%
Financing Receivable, Real Estate Owned [Member]
Schedule of Expected Losses to be Paid [Line Items]
Insured Financial Obligations, Projected Loss Assumptions, Liquidation Rate	100.00%	100.00%	100.00%