Expected Loss to be Paid - Liquidation Rates and Key Assumptions in Base Case Expected Loss First Lien RMBS (Details) - scenario	3 Months Ended	12 Months Ended
	Mar. 31, 2019	Dec. 31, 2017	Dec. 31, 2018
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%
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%
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%
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%
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%
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%
Financing Receivables, 60 to 89 Days Past Due [Member] | Subprime [Member]
Schedule of Expected Losses to be Paid [Line Items]
Liquidation Rate	45.00%		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%		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%
Financing Receivables, Equal to Greater than 90 Days Past Due [Member] | Subprime [Member]
Schedule of Expected Losses to be Paid [Line Items]
Liquidation Rate	50.00%		50.00%
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	50.00%		50.00%
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%
Financing Receivables, Bankruptcy [Member] | Subprime [Member]
Schedule of Expected Losses to be Paid [Line Items]
Liquidation Rate	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%
Financing Receivables, Bankruptcy [Member] | Option ARM [Member]
Schedule of Expected Losses to be Paid [Line Items]
Liquidation Rate	50.00%		50.00%
Financing Receivable, Foreclosure [Member] | Subprime [Member]
Schedule of Expected Losses to be Paid [Line Items]
Liquidation Rate	60.00%		60.00%
Financing Receivable, Foreclosure [Member] | Alt-A and Prime [Member]
Schedule of Expected Losses to be Paid [Line Items]
Liquidation Rate	60.00%		60.00%
Financing Receivable, Foreclosure [Member] | Option ARM [Member]
Schedule of Expected Losses to be Paid [Line Items]
Liquidation Rate	65.00%		65.00%
Financing Receivable, Real Estate Owned [Member]
Schedule of Expected Losses to be Paid [Line Items]
Liquidation Rate	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%
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%		70.00%
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%
United States [Member] | Alt-A and Prime [Member] | Minimum [Member] | RMBS [Member]
Schedule of Expected Losses to be Paid [Line Items]
Plateau CDR	1.00%		1.20%
Final CDR	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	10.70%		11.40%
Final CDR	0.50%		0.60%
United States [Member] | Alt-A and Prime [Member] | Weighted Average [Member] | RMBS [Member]
Schedule of Expected Losses to be Paid [Line Items]
Plateau CDR	4.30%		4.60%
Final CDR	0.20%		0.20%
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%
United States [Member] | Option ARM [Member] | 2006 [Member] | RMBS [Member]
Schedule of Expected Losses to be Paid [Line Items]
Initial loss severity	60.00%		60.00%
United States [Member] | Option ARM [Member] | 2007 [Member] | RMBS [Member]
Schedule of Expected Losses to be Paid [Line Items]
Initial loss severity	70.00%		70.00%
United States [Member] | Option ARM [Member] | Minimum [Member] | RMBS [Member]
Schedule of Expected Losses to be Paid [Line Items]
Plateau CDR	2.10%		1.80%
Final CDR	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	9.30%		8.30%
Final CDR	0.50%		0.40%
United States [Member] | Option ARM [Member] | Weighted Average [Member] | RMBS [Member]
Schedule of Expected Losses to be Paid [Line Items]
Plateau CDR	5.90%		5.60%
Final CDR	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%
United States [Member] | Subprime [Member] | 2006 [Member] | RMBS [Member]
Schedule of Expected Losses to be Paid [Line Items]
Initial loss severity	75.00%		75.00%
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%
United States [Member] | Subprime [Member] | Minimum [Member] | RMBS [Member]
Schedule of Expected Losses to be Paid [Line Items]
Plateau CDR	2.40%		1.80%
Final CDR	0.10%		0.10%
United States [Member] | Subprime [Member] | Maximum [Member] | RMBS [Member]
Schedule of Expected Losses to be Paid [Line Items]
Plateau CDR	24.10%		23.20%
Final CDR	1.20%		1.20%
United States [Member] | Subprime [Member] | Weighted Average [Member] | RMBS [Member]
Schedule of Expected Losses to be Paid [Line Items]
Plateau CDR	6.20%		6.20%
Final CDR	0.30%		0.30%
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
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 3 months
Final CPR	15.00%		15.00%