Significant Accounting Policies - Variable Interest Entities (Details)	3 Months Ended	12 Months Ended
	Mar. 31, 2023 USD ($)	Dec. 31, 2023 USD ($)	Jan. 31, 2023 USD ($)	Jan. 01, 2023 USD ($)	Dec. 31, 2022 USD ($)	Dec. 31, 2021 USD ($)	Dec. 31, 2020 USD ($)
Variable interest entity
Gain on consolidation of Sponsored REIT		$ 394,000
Cash, cash equivalents and restricted cash		127,880,000			$ 6,632,000	$ 40,751,000	$ 4,150,000
VIE at Consolidation
Real estate assets		890,824,000			1,103,248,000
Acquired real estate leases		6,694,000			10,186,000
Related party mortgage loan receivable					19,763,000
FSP Monument Circle LLC
Variable interest entity
Cash, cash equivalents and restricted cash		2,167,000
Level 3
VIE at Consolidation
Related party mortgage loan receivable					$ 19,763,000
Level 3 | Exit Cap Rate | Maximum
VIE at Consolidation
Measurement input					7.50
Level 3 | Exit Cap Rate | Minimum
VIE at Consolidation
Measurement input					7.50
Level 3 | Exit Cap Rate | Weighted Average
VIE at Consolidation
Measurement input					7.50
Level 3 | Discount Rate | Maximum
VIE at Consolidation
Measurement input					9.50
Level 3 | Discount Rate | Minimum
VIE at Consolidation
Measurement input					9.50
Level 3 | Discount Rate | Weighted Average
VIE at Consolidation
Measurement input					9.50
Variable interest entities ("VIEs")
Variable interest entity
Gain on consolidation of Sponsored REIT	$ 400,000
Cash, cash equivalents and restricted cash		2,167,000			$ 0
VIE at Consolidation
Real estate assets		19,354,000	$ 19,695,000		0
Acquired real estate leases		305,000	305,000		0
Total			$ 20,000,000
Variable interest entities ("VIEs") | FSP Monument Circle LLC
Variable interest entity
Cash, cash equivalents and restricted cash		$ 3,000,000
Variable interest entities ("VIEs") | Level 3
VIE at Consolidation
Related party mortgage loan receivable				$ (20,000,000)
Variable interest entities ("VIEs") | Level 3 | Exit Cap Rate | Maximum
VIE at Consolidation
Measurement input				(0.0750)
Variable interest entities ("VIEs") | Level 3 | Exit Cap Rate | Minimum
VIE at Consolidation
Measurement input				(0.0750)
Variable interest entities ("VIEs") | Level 3 | Exit Cap Rate | Weighted Average
VIE at Consolidation
Measurement input				(0.0750)
Variable interest entities ("VIEs") | Level 3 | Discount Rate | Maximum
VIE at Consolidation
Measurement input				(0.0950)
Variable interest entities ("VIEs") | Level 3 | Discount Rate | Minimum
VIE at Consolidation
Measurement input				(0.0950)
Variable interest entities ("VIEs") | Level 3 | Discount Rate | Weighted Average
VIE at Consolidation
Measurement input				(0.0950)
VIE, Not Primary Beneficiary
Variable interest entity
Maximum exposure to losses associated with VIE					22,100,000
VIE, Not Primary Beneficiary | Prepaid expenses and other assets
Variable interest entity
Accrued interest and exit fee receivables					$ 2,300,000