Fair Value of Financial Instruments - Carrying Value and Estimated Fair Value of Company's Financial Instruments (Details) - USD ($) $ in Thousands	Jun. 30, 2021	Dec. 31, 2020
Accounts, Notes, Loans and Financing Receivable [Line Items]
Participations sold	$ (27,662)	$ (34,974)
Commercial Mortgage Portfolio Segment
Accounts, Notes, Loans and Financing Receivable [Line Items]
Participations sold	(27,662)	(9,217)
Subordinate Mortgage Portfolio Segment
Accounts, Notes, Loans and Financing Receivable [Line Items]
Participations sold	0	(25,757)
Carrying Value
Accounts, Notes, Loans and Financing Receivable [Line Items]
Cash and cash equivalents	204,581	325,498
Carrying Value \| Level III
Accounts, Notes, Loans and Financing Receivable [Line Items]
Secured debt arrangements, net	(3,679,906)	(3,436,672)
Participations sold	(27,662)	(34,974)
Debt related To real estate owned, held for sale	0	(33,000)
Carrying Value \| Level III \| Secured Debt
Accounts, Notes, Loans and Financing Receivable [Line Items]
Senior secured term loans, net	(774,569)	(483,465)
Carrying Value \| Level III \| Senior Notes
Accounts, Notes, Loans and Financing Receivable [Line Items]
Senior secured term loans, net	(493,694)	0
Carrying Value \| Level III \| 2022 Notes
Accounts, Notes, Loans and Financing Receivable [Line Items]
Notes	(341,720)	(340,361)
Carrying Value \| Level III \| 2023 Notes
Accounts, Notes, Loans and Financing Receivable [Line Items]
Notes	(226,065)	(225,293)
Carrying Value \| Level III \| Commercial Mortgage Portfolio Segment
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	6,653,197	5,451,084
Carrying Value \| Level III \| Subordinate Mortgage Portfolio Segment
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	866,908	1,045,893
Estimate of Fair Value Measurement
Accounts, Notes, Loans and Financing Receivable [Line Items]
Cash and cash equivalents	204,581	325,498
Estimate of Fair Value Measurement \| Level III
Accounts, Notes, Loans and Financing Receivable [Line Items]
Secured debt arrangements, net	(3,679,906)	(3,436,672)
Participations sold	(27,695)	(34,919)
Debt related To real estate owned, held for sale	0	(33,000)
Estimate of Fair Value Measurement \| Level III \| Secured Debt
Accounts, Notes, Loans and Financing Receivable [Line Items]
Senior secured term loans, net	(784,112)	(475,263)
Estimate of Fair Value Measurement \| Level III \| Senior Notes
Accounts, Notes, Loans and Financing Receivable [Line Items]
Senior secured term loans, net	(496,145)	0
Estimate of Fair Value Measurement \| Level III \| 2022 Notes
Accounts, Notes, Loans and Financing Receivable [Line Items]
Notes	(359,904)	(327,381)
Estimate of Fair Value Measurement \| Level III \| 2023 Notes
Accounts, Notes, Loans and Financing Receivable [Line Items]
Notes	(234,747)	(214,875)
Estimate of Fair Value Measurement \| Level III \| Commercial Mortgage Portfolio Segment
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	6,508,986	5,361,948
Estimate of Fair Value Measurement \| Level III \| Subordinate Mortgage Portfolio Segment
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	$ 774,257	$ 1,027,582

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Fair value portion of notes payable.

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Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
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-Topic 210
-SubTopic 10
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The aggregate amount of the participation liability at the end of the accounting period.

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Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 470
-SubTopic 30
-Section 50
-Paragraph 1
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