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Financings - Narrative (Details)	1 Months Ended						12 Months Ended
Financings - Narrative (Details)	Aug. 31, 2019 USD ($)	May 31, 2019 USD ($)	Mar. 31, 2019 USD ($)	Mar. 31, 2018 USD ($)	Oct. 31, 2017 USD ($)	Nov. 30, 2015 USD ($)	Dec. 31, 2019 USD ($) item	Dec. 31, 2018 USD ($)	Sep. 30, 2019 USD ($)
Borrowings Under Repurchase Agreements
Number of counterparties to master repurchase agreement \| item							34
Number of counterparties from whom the Company had borrowings \| item							21
Certain characteristics of the Company's repurchase agreements
Repurchase Agreement Borrowings							$ 2,824,877,000	$ 2,818,837,000
Less unamortized debt issuance costs							76,000	0
Borrowings under repurchase agreements							$ 2,824,801,000	$ 2,818,837,000
Weighted Average Interest Rate on Borrowings Outstanding at end of period							2.61%	3.45%
Weighted Average Remaining Maturity (days)							179 days	54 days
Securitized debt, at fair value							$ 1,477,454,000	$ 949,626,000
Securitized debt							$ 1,057,000
Convertible Debt \| Six Point Seventy Five Percent Convertible Senior Unsecured Notes
Borrowings Under Repurchase Agreements
Convertible senior unsecured notes, earliest redemption period by company	3 months				3 months		3 months
Certain characteristics of the Company's repurchase agreements
Principal Balance	$ 40,000,000.0				$ 115,000,000.0		$ 205,000,000.0
Convertible Debt \| Three Point Seventy Five Percent Convertible Senior Unsecured Notes
Certain characteristics of the Company's repurchase agreements
Convertible senior unsecured notes, conversion ratio					0.0831947
Repurchase Agreements
Certain characteristics of the Company's repurchase agreements
Repurchase Agreement Borrowings							$ 535,991,000	$ 0
Long-term Debt, Weighted Average Interest Rate, at Point in Time							3.45%	0.00%
Weighted Average Remaining Maturity (days)							805 days	0 years
Whole-Loans and Securitized Commercial Loan \| Repurchase Agreements
Certain characteristics of the Company's repurchase agreements
Repurchase Agreement Borrowings							$ 374,143,000	$ 0
Long-term Debt, Weighted Average Interest Rate, at Point in Time							3.27%	0.00%
Weighted Average Remaining Maturity (days)							898 days	0 years
Commercial Loans \| Repurchase Agreements
Certain characteristics of the Company's repurchase agreements
Repurchase Agreement Borrowings							$ 161,848,000	$ 0
Long-term Debt, Weighted Average Interest Rate, at Point in Time							3.88%	0.00%
Weighted Average Remaining Maturity (days)							590 days	0 years
VIE
Certain characteristics of the Company's repurchase agreements
Securitized debt, at fair value							$ 1,477,454,000	$ 949,626,000
CMSC Trust
Certain characteristics of the Company's repurchase agreements
Commercial pass-through certificates, amount							25,000,000.0
Trust certificates issued							24,000,000.0
Financing receivable, gross							24,048,000	24,456,000
Trust certificates, at fair value							$ 24,100,000
Debt instrument, interest rate, effective percentage							8.90%
RETL Trust
Certain characteristics of the Company's repurchase agreements
Trust certificates issued							$ 598,500,000
Financing receivable, gross							674,331,000	988,609,000
RETL Trust \| Secured Debt
Certain characteristics of the Company's repurchase agreements
Principal Balance							674,331,000
Fair value							676,435,000
RETL Trust \| Secured Debt \| Class C
Certain characteristics of the Company's repurchase agreements
Principal Balance							308,400,000
Payments to acquire securitized debt							30,600,000
Fair value							309,171,000
RETL Trust \| Secured Debt \| Commercial Mortgage Pass-Through Certificate, Class A
Certain characteristics of the Company's repurchase agreements
Principal Balance							219,431,000
Fair value							219,567,000
RETL Trust \| Affiliated Entity
Certain characteristics of the Company's repurchase agreements
Trust certificates issued							60,600,000
RETL Trust \| Third Party
Certain characteristics of the Company's repurchase agreements
Trust certificates issued							537,900,000
Arroyo Trust \| Secured Debt
Certain characteristics of the Company's repurchase agreements
Principal Balance									$ 801,114,000
Fair value									795,811,000
MRCD Trust
Certain characteristics of the Company's repurchase agreements
Trust certificates issued							83,600,000
Financing receivable, gross							245,000,000.0	0
MRCD Trust \| Secured Debt
Certain characteristics of the Company's repurchase agreements
Principal Balance							245,000,000
Fair value							208,547,000
MRCD Trust \| Secured Debt \| Commercial Mortgage Pass-Through Certificate, Class A
Certain characteristics of the Company's repurchase agreements
Principal Balance							234,500,000
Payments to acquire securitized debt							150,900,000
Fair value							198,104,000
Repurchase Agreements
Certain characteristics of the Company's repurchase agreements
Repurchase Agreement Borrowings							$ 2,288,886,000	$ 2,818,837,000
Weighted Average Interest Rate on Borrowings Outstanding at end of period							2.41%	3.45%
Weighted Average Remaining Maturity (days)							32 days	54 days
Repurchase Agreements \| Subtotal Agency RMBS
Certain characteristics of the Company's repurchase agreements
Repurchase Agreement Borrowings							$ 348,274,000	$ 14,650,000
Weighted Average Interest Rate on Borrowings Outstanding at end of period							1.99%	3.09%
Weighted Average Remaining Maturity (days)							52 days	21 days
Repurchase Agreements \| Agency CMBS
Certain characteristics of the Company's repurchase agreements
Repurchase Agreement Borrowings							$ 1,352,248,000	$ 1,392,649,000
Weighted Average Interest Rate on Borrowings Outstanding at end of period							2.05%	2.71%
Weighted Average Remaining Maturity (days)							26 days	40 days
Repurchase Agreements \| Non-Agency RMBS
Certain characteristics of the Company's repurchase agreements
Repurchase Agreement Borrowings							$ 30,481,000	$ 30,922,000
Weighted Average Interest Rate on Borrowings Outstanding at end of period							3.56%	4.06%
Weighted Average Remaining Maturity (days)							9 days	18 days
Repurchase Agreements \| Non-Agency CMBS
Certain characteristics of the Company's repurchase agreements
Repurchase Agreement Borrowings							$ 190,390,000	$ 134,814,000
Weighted Average Interest Rate on Borrowings Outstanding at end of period							3.05%	4.05%
Weighted Average Remaining Maturity (days)							35 days	48 days
Repurchase Agreements \| Whole-Loans and Securitized Commercial Loan
Certain characteristics of the Company's repurchase agreements
Repurchase Agreement Borrowings							$ 102,029,000	$ 863,356,000
Weighted Average Interest Rate on Borrowings Outstanding at end of period							3.51%	4.08%
Weighted Average Remaining Maturity (days)							27 days	93 days
Repurchase Agreements \| Residential Bridge Loans
Certain characteristics of the Company's repurchase agreements
Repurchase Agreement Borrowings							$ 29,869,000	$ 204,754,000
Weighted Average Interest Rate on Borrowings Outstanding at end of period							3.93%	4.50%
Weighted Average Remaining Maturity (days)							28 days	25 days
Repurchase Agreements \| Commercial Loans
Certain characteristics of the Company's repurchase agreements
Repurchase Agreement Borrowings							$ 62,746,000	$ 131,788,000
Weighted Average Interest Rate on Borrowings Outstanding at end of period							4.04%	4.55%
Weighted Average Remaining Maturity (days)							28 days	26 days
Repurchase Agreements \| Securitized Commercial Loans
Certain characteristics of the Company's repurchase agreements
Repurchase Agreement Borrowings							$ 116,087,000	$ 7,543,000
Weighted Average Interest Rate on Borrowings Outstanding at end of period							3.93%	4.30%
Weighted Average Remaining Maturity (days)							49 days	15 days
Repurchase Agreements \| Other securities
Certain characteristics of the Company's repurchase agreements
Repurchase Agreement Borrowings							$ 56,762,000	$ 38,361,000
Weighted Average Interest Rate on Borrowings Outstanding at end of period							3.23%	4.18%
Weighted Average Remaining Maturity (days)							34 days	26 days
Securitized Commercial Loans \| VIE
Certain characteristics of the Company's repurchase agreements
Securitized debt							$ 681,643,000	$ 949,626,000
Securitized Commercial Loans \| CMSC Trust
Certain characteristics of the Company's repurchase agreements
Amount acquired, eliminated in consolidation						$ 14,000,000.0	13,500,000
Securitized debt, at fair value							10,600,000
Securitized Commercial Loans \| CMSC Trust \| Securitized Debt
Certain characteristics of the Company's repurchase agreements
Securitized debt, at fair value							10,600,000
Securitized Commercial Loans \| RETL Trust
Certain characteristics of the Company's repurchase agreements
Amount acquired, eliminated in consolidation			$ 65,300,000	$ 67,800,000
Securitized Commercial Loans \| MRCD Trust
Certain characteristics of the Company's repurchase agreements
Amount acquired, eliminated in consolidation							161,400,000
Residential Whole Loans, at Fair Value \| VIE
Certain characteristics of the Company's repurchase agreements
Borrowings under repurchase agreements							209,900,000	618,700,000
Residential Whole Loans, at Fair Value \| Arroyo Mortgage Trust 2019-2
Certain characteristics of the Company's repurchase agreements
Borrowings under repurchase agreements							919,000,000.0		919,000,000.0
Residential Whole Loans, at Fair Value \| Arroyo Mortgage Trust 2019-2 \| Arroyo Trust
Certain characteristics of the Company's repurchase agreements
Transfer of residential mortgage-backed securitization		$ 945,500,000
Residential Whole Loans, at Fair Value \| Arroyo Trust \| Secured Debt
Certain characteristics of the Company's repurchase agreements
Fair value							$ 814,000,000.0
Debt instrument, collateral balance percentage							20.00%
Residential Facility
Certain characteristics of the Company's repurchase agreements
Line of credit facility, maximum borrowing capacity									$ 700,000,000.0
Commercial Facility
Certain characteristics of the Company's repurchase agreements
Line of credit facility, maximum borrowing capacity							$ 200.0
Estimate of Fair Value Measurement
Certain characteristics of the Company's repurchase agreements
Securitized debt							681,643,000	$ 949,626,000
Estimate of Fair Value Measurement \| RETL Trust \| Secured Debt \| Class C
Certain characteristics of the Company's repurchase agreements
Securitized debt							600,500,000
Estimate of Fair Value Measurement \| MRCD Trust \| Secured Debt \| Commercial Mortgage Pass-Through Certificate, Class A
Certain characteristics of the Company's repurchase agreements
Securitized debt							$ 70,600,000