Financings - Borrowings under Repurchase Agreements (Details)	3 Months Ended	6 Months Ended	12 Months Ended
Mar. 31, 2020 USD ($) agreement	Jun. 30, 2019	Dec. 31, 2019 USD ($)
Certain characteristics of the Company's repurchase agreements
Repurchase Agreement Borrowings	$ 1,553,781,000		$ 2,824,877,000
Less unamortized debt issuance costs	66,000		76,000
Repurchase agreements, net	$ 1,553,715,000		$ 2,824,801,000
Weighted Average Interest Rate on Borrowings Outstanding at end of period	2.50%		2.61%
Weighted Average Remaining Maturity (days)	271 days	179 days	179 days
Weighted Average Interest Rate on Borrowings Outstanding at end of period, Total	2.50%		2.61%
Number of long-term repurchase agreements \| agreement	2
Repurchase agreements
Certain characteristics of the Company's repurchase agreements
Repurchase Agreement Borrowings	$ 438,958,000		$ 371,726,000
Weighted Average Remaining Maturity (days)	829 days		1024 days
Weighted Average Interest Rate on Borrowings Outstanding at end of period	2.70%		3.70%
Repurchase agreements \| Whole-Loans
Certain characteristics of the Company's repurchase agreements
Repurchase Agreement Borrowings	$ 285,409,000		$ 209,878,000
Weighted Average Remaining Maturity (days)	1004 days		1358 days
Weighted Average Interest Rate on Borrowings Outstanding at end of period	2.67%		3.55%
Repurchase agreements \| Commercial Loans
Certain characteristics of the Company's repurchase agreements
Repurchase Agreement Borrowings	$ 153,549,000		$ 161,848,000
Weighted Average Remaining Maturity (days)	503 days		590 days
Weighted Average Interest Rate on Borrowings Outstanding at end of period	2.73%		3.88%
Repurchase agreements
Certain characteristics of the Company's repurchase agreements
Repurchase Agreement Borrowings	$ 1,114,823,000		$ 2,453,151,000
Weighted Average Interest Rate on Borrowings Outstanding at end of period	2.42%		2.44%
Weighted Average Remaining Maturity (days)	51 days	51 days
Repurchase agreements \| Subtotal Agency
Certain characteristics of the Company's repurchase agreements
Repurchase Agreement Borrowings	$ 437,577,000		$ 1,352,248,000
Weighted Average Interest Rate on Borrowings Outstanding at end of period	1.38%		2.05%
Weighted Average Remaining Maturity (days)	27 days	26 days
Repurchase agreements \| Agency CMBS
Certain characteristics of the Company's repurchase agreements
Repurchase Agreement Borrowings	$ 11,852,000		$ 348,274,000
Weighted Average Interest Rate on Borrowings Outstanding at end of period	2.35%		1.99%
Weighted Average Remaining Maturity (days)	21 days	52 days
Repurchase agreements \| Non-Agency RMBS
Certain characteristics of the Company's repurchase agreements
Repurchase Agreement Borrowings	$ 214,972,000		$ 190,390,000
Weighted Average Interest Rate on Borrowings Outstanding at end of period	3.04%		3.05%
Weighted Average Remaining Maturity (days)	24 days	35 days
Repurchase agreements \| Non-Agency CMBS
Certain characteristics of the Company's repurchase agreements
Repurchase Agreement Borrowings	$ 20,148,000		$ 30,481,000
Weighted Average Interest Rate on Borrowings Outstanding at end of period	3.09%		3.56%
Weighted Average Remaining Maturity (days)	8 days	9 days
Repurchase agreements \| Whole-Loans
Certain characteristics of the Company's repurchase agreements
Repurchase Agreement Borrowings	$ 272,458,000		$ 266,294,000
Weighted Average Interest Rate on Borrowings Outstanding at end of period	2.99%		3.14%
Weighted Average Remaining Maturity (days)	129 days	202 days
Repurchase agreements \| Residential Bridge Loans
Certain characteristics of the Company's repurchase agreements
Repurchase Agreement Borrowings	$ 24,222,000		$ 29,869,000
Weighted Average Interest Rate on Borrowings Outstanding at end of period	3.79%		3.93%
Weighted Average Remaining Maturity (days)	28 days	28 days
Repurchase agreements \| Commercial Loans
Certain characteristics of the Company's repurchase agreements
Repurchase Agreement Borrowings	$ 47,547,000		$ 62,746,000
Weighted Average Interest Rate on Borrowings Outstanding at end of period	3.90%		4.04%
Weighted Average Remaining Maturity (days)	28 days	28 days
Repurchase agreements \| Securitized commercial loans
Certain characteristics of the Company's repurchase agreements
Repurchase Agreement Borrowings	$ 32,803,000		$ 116,087,000
Weighted Average Interest Rate on Borrowings Outstanding at end of period	2.76%		3.93%
Weighted Average Remaining Maturity (days)	29 days	49 days
Repurchase agreements \| Other securities
Certain characteristics of the Company's repurchase agreements
Repurchase Agreement Borrowings	$ 53,244,000		$ 56,762,000
Weighted Average Interest Rate on Borrowings Outstanding at end of period	3.15%		3.23%
Weighted Average Remaining Maturity (days)	28 days	34 days
Residential Facility
Certain characteristics of the Company's repurchase agreements
Maximum borrowing capacity	$ 700,000,000.0
Commercial Facility
Certain characteristics of the Company's repurchase agreements
Maximum borrowing capacity	$ 200,000,000.0

Maximum borrowing capacity under the credit facility without consideration of any current restrictions on the amount that could be borrowed or the amounts currently outstanding under the facility.

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