Expected Loss to be Paid - Key Assumptions in Base Case Expected Loss Second 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
RMBS [Member] | United States [Member] | Option ARM [Member] | Minimum [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%
RMBS [Member] | United States [Member] | Option ARM [Member] | Maximum [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%
RMBS [Member] | United States [Member] | Option ARM [Member] | Weighted Average [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%
RMBS [Member] | United States [Member] | Alt-A and Prime [Member] | Minimum [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%
RMBS [Member] | United States [Member] | Alt-A and Prime [Member] | Maximum [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%
RMBS [Member] | United States [Member] | Alt-A and Prime [Member] | Weighted Average [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%
RMBS [Member] | United States [Member] | Home Equity Line of Credit [Member]
Schedule of Expected Losses to be Paid [Line Items]
Loss severity	98.00%	98.00%	98.00%	98.00%
RMBS [Member] | United States [Member] | Home Equity Line of Credit [Member] | Minimum [Member]
Schedule of Expected Losses to be Paid [Line Items]
Insured Financial Obligations, Projected Loss Assumptions, Conditional Prepayment Rate, Final Rate	2.50%		2.50%
Plateau CDR	2.90%	4.80%	2.90%	2.70%
Final CDR	2.50%	2.50%	2.50%	2.50%
RMBS [Member] | United States [Member] | Home Equity Line of Credit [Member] | Maximum [Member]
Schedule of Expected Losses to be Paid [Line Items]
Plateau CDR	31.70%	28.50%	31.70%	19.90%
Final CDR	3.20%	3.20%	3.20%	3.20%
RMBS [Member] | United States [Member] | Home Equity Line of Credit [Member] | Weighted Average [Member]
Schedule of Expected Losses to be Paid [Line Items]
Plateau CDR	11.50%	11.10%	11.50%	11.40%
Final CDR	2.50%	2.50%	2.50%	2.50%
RMBS [Member] | United States [Member] | Subprime [Member] | Minimum [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%
RMBS [Member] | United States [Member] | Subprime [Member] | Maximum [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%
RMBS [Member] | United States [Member] | Subprime [Member] | Weighted Average [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%
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] | RMBS [Member] | United States [Member] | Home Equity Line of Credit [Member]
Schedule of Expected Losses to be Paid [Line Items]
Insured Financial Obligations, Projected Loss Assumptions, Liquidation Rate	20.00%	20.00%	20.00%	20.00%
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] | RMBS [Member] | United States [Member] | Home Equity Line of Credit [Member]
Schedule of Expected Losses to be Paid [Line Items]
Insured Financial Obligations, Projected Loss Assumptions, Liquidation Rate	40.00%	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	35.00%	35.00%	35.00%	40.00%
Financing Receivables, 60 to 89 Days Past Due [Member] | RMBS [Member] | United States [Member] | Home Equity Line of Credit [Member]
Schedule of Expected Losses to be Paid [Line Items]
Insured Financial Obligations, Projected Loss Assumptions, Liquidation Rate	55.00%	55.00%	55.00%	60.00%
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] | RMBS [Member] | United States [Member] | Home Equity Line of Credit [Member]
Schedule of Expected Losses to be Paid [Line Items]
Insured Financial Obligations, Projected Loss Assumptions, Liquidation Rate	75.00%	75.00%	75.00%	75.00%
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] | RMBS [Member] | United States [Member] | Home Equity Line of Credit [Member]
Schedule of Expected Losses to be Paid [Line Items]
Insured Financial Obligations, Projected Loss Assumptions, Liquidation Rate	55.00%	55.00%	55.00%	55.00%
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] | RMBS [Member] | United States [Member] | Home Equity Line of Credit [Member]
Schedule of Expected Losses to be Paid [Line Items]
Insured Financial Obligations, Projected Loss Assumptions, 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%
Financing Receivable, Real Estate Owned [Member] | RMBS [Member] | United States [Member] | Home Equity Line of Credit [Member]
Schedule of Expected Losses to be Paid [Line Items]
Insured Financial Obligations, Projected Loss Assumptions, Liquidation Rate	100.00%	100.00%	100.00%	100.00%
Second Lien [Member] | RMBS [Member] | United States [Member]
Schedule of Expected Losses to be Paid [Line Items]
Loss recovery assumption (as a percent)	2.00%	2.00%	2.00%	2.00%
Number of scenarios weighted in estimating expected losses				5
Second Lien [Member] | RMBS [Member] | United States [Member] | Home Equity Line of Credit and Closed-end Mortgage [Member]
Schedule of Expected Losses to be Paid [Line Items]
Insured Financial Obligations, Projected Loss Assumptions, Conditional Prepayment Rate, Final Rate	15.00%	15.00%	15.00%	15.00%
First Lien [Member] | RMBS [Member] | United States [Member]
Schedule of Expected Losses to be Paid [Line Items]
Number of scenarios weighted in estimating expected losses	5	5	5	5
Projected loss assumptions, Final CDR, Period for voluntary prepayments to continue			12 months
Base Scenario [Member] | First Lien [Member] | RMBS [Member] | United States [Member]
Schedule of Expected Losses to be Paid [Line Items]
Insured Financial Obligations, Projected Loss Assumptions, Conditional Prepayment Rate, Final Rate	15.00%	15.00%	15.00%	15.00%
Projected loss assumptions, Final CDR, Period for voluntary prepayments to continue			4 years 9 months
2005 and prior [Member] | RMBS [Member] | United States [Member] | Option ARM [Member]
Schedule of Expected Losses to be Paid [Line Items]
Loss severity	60.00%	60.00%	60.00%	60.00%
2005 and prior [Member] | RMBS [Member] | United States [Member] | Alt-A and Prime [Member]
Schedule of Expected Losses to be Paid [Line Items]
Loss severity	60.00%	60.00%	60.00%	60.00%
2005 and prior [Member] | RMBS [Member] | United States [Member] | Subprime [Member]
Schedule of Expected Losses to be Paid [Line Items]
Loss severity	80.00%	80.00%	80.00%	80.00%
2006 [Member] | RMBS [Member] | United States [Member] | Option ARM [Member]
Schedule of Expected Losses to be Paid [Line Items]
Loss severity	60.00%	70.00%	60.00%	70.00%
2006 [Member] | RMBS [Member] | United States [Member] | Alt-A and Prime [Member]
Schedule of Expected Losses to be Paid [Line Items]
Loss severity	70.00%	80.00%	70.00%	80.00%
2006 [Member] | RMBS [Member] | United States [Member] | Subprime [Member]
Schedule of Expected Losses to be Paid [Line Items]
Loss severity	75.00%	85.00%	75.00%	90.00%
2007 [Member] | RMBS [Member] | United States [Member] | Option ARM [Member]
Schedule of Expected Losses to be Paid [Line Items]
Loss severity	70.00%	75.00%	70.00%	75.00%
2007 [Member] | RMBS [Member] | United States [Member] | Alt-A and Prime [Member]
Schedule of Expected Losses to be Paid [Line Items]
Loss severity	70.00%	70.00%	70.00%	70.00%
2007 [Member] | RMBS [Member] | United States [Member] | Subprime [Member]
Schedule of Expected Losses to be Paid [Line Items]
Loss severity	95.00%	95.00%	95.00%	95.00%