Other-Than-Temporary Impairment Analysis (Significant Inputs for Other-Than-Temporarily Impaired PLRMBS) (Details)	12 Months Ended
	Dec. 31, 2019	[1]
Other than Temporary Impairment, Credit Losses Recognized in Earnings [Line Items]
Prepayment Weighted Average	12.70%
Default Rate Weighted Average	20.10%
Loss Severity Weighted Average	44.10%
Credit Enhancements Weighted Average	1.10%
Prime | Residential Mortgage Backed Securities
Other than Temporary Impairment, Credit Losses Recognized in Earnings [Line Items]
Prepayment Weighted Average	15.50%
Default Rate Weighted Average	11.10%
Loss Severity Weighted Average	18.70%
Credit Enhancements Weighted Average	31.90%
Alt-A | Residential Mortgage Backed Securities
Other than Temporary Impairment, Credit Losses Recognized in Earnings [Line Items]
Prepayment Weighted Average	12.70%
Default Rate Weighted Average	20.10%
Loss Severity Weighted Average	44.10%
Credit Enhancements Weighted Average	1.10%

[1] Weighted average percentage is based on unpaid principal balance.