Note 8 - Debt - Summary of Financing Arrangements (Details) - USD ($)		9 Months Ended	12 Months Ended
		Sep. 30, 2021	Dec. 31, 2020	Apr. 20, 2021
Unsecured Debt [Member]
Weighted average interest rate	[1]	7.50%	7.50%
Facility [Member]
Outstanding face amount		$ 1,139,460	$ 1,038,418
Carrying value		$ 1,135,990	$ 1,036,878
Weighted average interest rate	[1]	2.59%	2.50%
Weighted average life (Year)	[2]	5 years 3 months 14 days	6 years 3 months 7 days
Facility [Member] | Unsecured Debt [Member]
Outstanding face amount		$ 36,500	$ 36,500
Carrying value		$ 35,160	$ 34,960
Weighted average life (Year)	[2]	4 years 1 month 6 days	4 years 9 months 18 days
Collateral [Member]
Collateral outstanding amount		$ 2,861,283	$ 2,908,143
Amortized cost basis		1,364,665	1,332,803
Collateral carrying value	[3]	$ 1,395,534	$ 1,335,998
Collateral weighted average life (Year)	[2]	8 years 3 days	9 years 6 months 7 days
Master Repurchase Agreements Collateralized By CMBS[Member] | Mizuho [Member]
Outstanding face amount	[4]	$ 222,533	$ 161,465
Carrying value	[4]	$ 222,533	$ 161,465
Weighted average interest rate	[1],[4]	1.91%	2.46%
Weighted average life (Year)	[2],[4]	18 days	7 days
Collateral outstanding amount	[4]	$ 1,947,301	$ 1,955,879
Amortized cost basis	[4]	392,031	313,632
Collateral carrying value	[3],[4]	$ 422,900	$ 316,827
Collateral weighted average life (Year)	[2],[4]	8 years 6 months	10 years 7 months 6 days
Asset Specific Financing [Member] | Freddie Mac [Member] | Facility [Member]
Outstanding face amount		$ 745,513	$ 780,539
Carrying value		$ 745,513	$ 780,539
Weighted average interest rate	[1]	2.42%	2.44%
Weighted average life (Year)	[2]	6 years 8 months 12 days	7 years 4 months 24 days
Asset Specific Financing [Member] | Freddie Mac [Member] | Collateral [Member]
Collateral outstanding amount		$ 816,083	$ 854,365
Amortized cost basis		871,711	918,114
Collateral carrying value	[3]	$ 871,711	$ 918,114
Collateral weighted average life (Year)	[2]	6 years 8 months 12 days	7 years 4 months 24 days
Mezzanine Loan [Member] | Freddie Mac [Member] | Facility [Member]
Outstanding face amount		$ 59,914	$ 59,914
Carrying value		$ 59,914	$ 59,914
Weighted average interest rate	[1]	0.30%	0.30%
Weighted average life (Year)	[2]	8 years 7 months 6 days	9 years 3 months 18 days
Mezzanine Loan [Member] | Freddie Mac [Member] | Collateral [Member]
Collateral outstanding amount		$ 97,899	$ 97,899
Amortized cost basis		100,923	101,057
Collateral carrying value	[3]	$ 100,923	$ 101,057
Collateral weighted average life (Year)	[2]	8 years 7 months 6 days	7 years 1 month 6 days
The 5.75 Percent Senior Notes Due 2026 [Member] | Unsecured Debt [Member]
Outstanding face amount				$ 75,000,000
Weighted average interest rate	[1]	5.75%
The 5.75 Percent Senior Notes Due 2026 [Member] | Facility [Member] | Unsecured Debt [Member]
Outstanding face amount		$ 75,000
Carrying value		$ 72,870
Weighted average life (Year)	[2]	4 years 6 months

[1] Weighted-average interest rate using unpaid principal balances.

[2] Weighted-average life is determined using the maximum maturity date of the corresponding loans, assuming all extension options are exercised by the borrower.

[3] CMBS are shown at fair value. SFR Loans and mezzanine loans are shown at their carrying values.

[4] On April 15, 2020, three of our subsidiaries entered into a master repurchase agreement with Mizuho Securities ("Mizuho"). Borrowings under these repurchase agreements are collateralized by portions of the CMBS B-Pieces and CMBS I/O Strips.