Fair Value Measurements - Financial Instruments at Fair Value (Details) - Mortgages - USD ($) $ in Thousands	Dec. 31, 2021	Dec. 31, 2020
Fair Value | Fair Value, Inputs, Level 2
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Mortgage loans	$ 995,645	$ 1,016,565
Fair Value | BOA Loan | Fair Value, Inputs, Level 2
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Mortgage loans	349,082	355,823
Fair Value | BOA/KeyBank Loan | Fair Value, Inputs, Level 2
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Mortgage loans	260,378	263,454
Fair Value | AIG Loan II | Fair Value, Inputs, Level 2
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Mortgage loans	120,141	121,011
Fair Value | AIG Loan | Fair Value, Inputs, Level 2
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Mortgage loans	99,697	102,033
Fair Value | Midland Mortgage Loan | Fair Value, Inputs, Level 2
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Mortgage loans	95,720	97,709
Fair Value | Samsonite Loan | Fair Value, Inputs, Level 2
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Mortgage loans	19,366	21,030
Fair Value | HealthSpring Mortgage Loan | Fair Value, Inputs, Level 2
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Mortgage loans	19,639	20,462
Fair Value | CCIT II Properties | Fair Value, Inputs, Level 2
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Mortgage loans	18,262	18,942
Fair Value | Highway 94 Loan | Fair Value, Inputs, Level 2
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Mortgage loans	13,360	14,447
Fair Value | Emporia Partners Mortgage Loan | Fair Value, Inputs, Level 2
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Mortgage loans	0	1,654
Carrying Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Mortgage loans	1,018,015	1,029,093
Carrying Value | BOA Loan
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Mortgage loans	375,000	375,000
Carrying Value | BOA/KeyBank Loan
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Mortgage loans	250,000	250,000
Carrying Value | AIG Loan II
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Mortgage loans	124,606	126,792
Carrying Value | AIG Loan
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Mortgage loans	101,884	103,870
Carrying Value | Midland Mortgage Loan
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Mortgage loans	95,792	98,155
Carrying Value | Samsonite Loan
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Mortgage loans	19,114	20,165
Carrying Value | HealthSpring Mortgage Loan
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Mortgage loans	19,669	20,208
Carrying Value | CCIT II Properties
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Mortgage loans	18,218	18,587
Carrying Value | Highway 94 Loan
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Mortgage loans	13,732	14,689
Carrying Value | Emporia Partners Mortgage Loan
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Mortgage loans	$ 0	$ 1,627