Residential Mortgage Securities and MSR Related Assets (MSR-Related Assets - Narrative) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Term Notes backed by MSR Related Collateral
Debt Securities, Available-for-sale [Line Items]
Term notes backed by MSR related collateral	$ 238,999	$ 1,157,463
Amortized costs	184,900	1,200,000
Gross unrealized gains	$ 54,000	$ 5,200
Weighted average yield	12.30%	4.75%
Weighted average to maturity	9 years 2 months 12 days	5 years 3 months 18 days
Proceeds from Sale of Mortgage Servicing Rights (MSR)	$ 711,700
Gross realized losses	28,700
Impairment	280,800
Corporate Loan
Debt Securities, Available-for-sale [Line Items]
Commitment to lend		$ 32,600
Amount drawn	$ 0
Minimum | Corporate Loan
Debt Securities, Available-for-sale [Line Items]
Commitment fee income rate	0.25%
Maximum | Corporate Loan
Debt Securities, Available-for-sale [Line Items]
Commitment fee income rate	1.00%