Agreements and transactions with related parties (Details)	12 Months Ended
	Dec. 31, 2020 USD ($) item	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)
Related-party transactions
Amount unpaid	$ 4,088,000	$ 2,697,000
Purchase of loans, held-for-investment	242,532,000	739,002,000	$ 369,418,000
Investment in unconsolidated joint ventures	$ 79,509,000	58,850,000
Management agreement
Related-party transactions
Automatically renewal period	1 year
Minimum notice period for termination	180 days
Termination fee multiplier	3
Period immediately preceding the termination used as basis for determination of the termination fee due	24 months
Management fee
Related-party transactions
Fee percentage for results up to threshold	1.50%
Fee threshold	$ 500,000,000
Fee percentage for results in excess of threshold	1.00%
Temporary reduction in the quarterly base management fee following the effective date of the merger	$ 1,000,000
Number of quarters the temporary reduction in quarterly base management fee is effective | item	4
Incentive distribution
Related-party transactions
Incentive multiplier	15.00%
Core earnings period	12 months
Percentage of Incentive fee multiplied by the weighted average of issue price	8.00%
The period over which core earnings must exceed the minimum threshold per the terms of the agreement	48 months
Minimum core earnings threshold	$ 0
Minimum | Management agreement
Related-party transactions
Independent director votes required for approval	0.667%
Manager | Management fee
Related-party transactions
Fees	$ 10,700,000	9,600,000
Amount unpaid	2,700,000	2,600,000
Manager | Incentive distribution
Related-party transactions
Fees	6,000,000.0	100,000
Amount unpaid	1,300,000	100,000
Manager | Expense reimbursement
Related-party transactions
Reimbursable expenses	8,400,000	7,000,000.0
Amount unpaid	$ 5,300,000	$ 4,200,000