Fair Value Measurements (Details) $ in Thousands	Sep. 30, 2021 USD ($) item	Dec. 31, 2020 USD ($) item
Fair value measurements
Mortgage loans receivable, net of loan loss reserve	$ 258,829	$ 257,251
Mortgage loans receivable, net of loan loss reserve	18,675	14,465
Senior unsecured notes, net of debt issue costs	$ 527,429	$ 559,482
Level 3 | Senior Unsecured Notes maturing before 2030 | Discount Rate
Fair value measurements
Future cash outflows discount rate (as a percent) | item	0.0300
Level 3 | Senior Unsecured Notes maturing 2030 and after | Discount Rate
Fair value measurements
Future cash outflows discount rate (as a percent) | item	0.0325
Level 3 | Senior Unsecured Notes maturing before 2026 | Discount Rate
Fair value measurements
Future cash outflows discount rate (as a percent) | item		0.0325
Level 3 | Senior Unsecured Notes maturing 2026 and after | Discount Rate
Fair value measurements
Future cash outflows discount rate (as a percent) | item		0.0350
Level 3 | Mortgage Loans Receivable | Discount Rate
Fair value measurements
Future cash inflows discount rate (as a percent)	0.101	0.100
Level 3 | Notes Receivable | Discount Rate
Fair value measurements
Future cash inflows discount rate (as a percent) | item	0.065	0.066
Carrying Value
Fair value measurements
Mortgage loans receivable, net of loan loss reserve	$ 258,829	$ 257,251
Mortgage loans receivable, net of loan loss reserve	18,675	14,465
Bank borrowings	134,400	89,900
Senior unsecured notes, net of debt issue costs	527,429	559,482
Fair Value
Fair value measurements
Mortgage loans receivable, net of loan loss reserve	18,552	13,893
Bank borrowings	134,400	89,900
Fair Value | Level 3
Fair value measurements
Mortgage loans receivable, net of loan loss reserve	298,677	299,751
Senior unsecured notes, net of debt issue costs	$ 556,841	$ 560,140