Secured Financing Agreements - Schedule of Secured Financing Agreements (Details) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020
Assets Sold under Agreements to Repurchase [Line Items]
Secured financing agreements outstanding secured by:	$ 3,800,000	$ 4,600,000
Debt issuance costs, net	3,000	8,000
Repurchase Agreements
Assets Sold under Agreements to Repurchase [Line Items]
Secured financing agreements outstanding secured by:	3,788,336	4,636,847
MBS pledged as collateral at fair value on Repurchase agreements:	5,018,442	6,696,214
Average balance of Secured financing agreements secured by:	4,110,855	7,316,345
Repurchase Agreements \| Agency RMBS
Assets Sold under Agreements to Repurchase [Line Items]
Secured financing agreements outstanding secured by:	24,700	69,180
MBS pledged as collateral at fair value on Repurchase agreements:	26,618	86,160
Average balance of Secured financing agreements secured by:	$ 54,242	$ 1,407,713
Average borrowing rate of Secured financing agreements secured by:	0.67%	0.90%
Average remaining maturity of Secured financing agreements secured by:	4 days	12 days
Average original maturity of Secured financing agreements secured by:	32 days	14 days
Repurchase Agreements \| Agency CMBS
Assets Sold under Agreements to Repurchase [Line Items]
Secured financing agreements outstanding secured by:	$ 795,873	$ 1,333,799
MBS pledged as collateral at fair value on Repurchase agreements:	835,984	1,382,783
Average balance of Secured financing agreements secured by:	$ 1,040,018	$ 1,818,721
Average borrowing rate of Secured financing agreements secured by:	0.16%	0.21%
Average remaining maturity of Secured financing agreements secured by:	13 days	11 days
Average original maturity of Secured financing agreements secured by:	30 days	30 days
Repurchase Agreements \| Non-agency MBS and Loans held for investment
Assets Sold under Agreements to Repurchase [Line Items]
Secured financing agreements outstanding secured by:	$ 2,967,763	$ 3,233,868
MBS pledged as collateral at fair value on Repurchase agreements:	4,155,840	5,227,271
Average balance of Secured financing agreements secured by:	$ 3,016,595	$ 4,089,911
Average borrowing rate of Secured financing agreements secured by:	2.76%	4.78%
Average remaining maturity of Secured financing agreements secured by:	316 days	458 days
Average original maturity of Secured financing agreements secured by:	333 days	492 days

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Repurchase Agreement Counterparty, Remaining Weighted Average Maturity of Agreements

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Amount, after accumulated amortization, of debt issuance costs. Includes, but is not limited to, legal, accounting, underwriting, printing, and registration costs.

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Amount, before effects of master netting arrangements, of funds outstanding borrowed in the form of a security repurchase agreement between the entity and another party for the sale and repurchase of identical or substantially the same securities at a date certain for a specified price. Includes liabilities elected not to be offset. Excludes liabilities not subject to a master netting arrangement.

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Fair value of financial instrument held by the entity for its own account (proprietary securities) for trading or investment purposes that are carried at fair value and pledged to counterparties as collateral for financing transactions.

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Weighted average interest rate of short-term debt outstanding calculated at point in time.

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For the form of debt having an initial term of less than one year or less than the normal operating cycle, if longer, average borrowings during the period.

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