Shareholders' Equity - Share Awards (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2021 USD ($) trustee installment shares	Dec. 31, 2020 USD ($) trustee shares	Dec. 31, 2019 USD ($) trustee shares	Dec. 31, 2018 shares
Officers and Other Employees
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Issuance of shares, net (in shares)	117,800	108,600	103,100
Issuance of shares, net | $	$ 2,994	$ 2,502	$ 3,080
2009 Award Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares granted (in shares)	145,800	136,600	136,100
Number of installments | installment	5
Unvested shares (in shares)	182,224	157,521	106,680	55,321
2022 (in shares)	60,184
2023 (in shares)	56,900
2024 (in shares)	41,580
2025 (in shares)	23,560
Estimated future compensation expense for the unvested shares | $	$ 4,279
Weighted average period of recognition of compensation expenses (in years)	23 months
Compensation expense | $	$ 2,868	$ 3,315	$ 3,088
Shares available for issuance under plan (in shares)	987,610
2009 Award Plan | Trustees
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of trustees | trustee	8	8	8
Shares granted (in shares)	3,500	3,500	3,000
Aggregate market value of shares awarded | $	$ 837	$ 745	$ 575
Market value of common shares awarded to each trustee (in dollars) | $	$ 105	$ 93	$ 72
2009 Award Plan | Managing Trustee | Common Shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of trustees | trustee			3
Shares granted (in shares)			3,000
Aggregate market value of shares awarded | $			$ 270
Market value of common shares awarded to each trustee (in dollars) | $			$ 90