Shareholders' Equity - Share-based Compensation (Details) - USD ($) $ / shares in Units, $ in Thousands		12 Months Ended
	Apr. 12, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Share Award Plan
Unvested shares, scheduled to vest
Shares reserved for issuance (in shares)		2,290,692
Award Plans
Number of Shares
Unvested shares, beginning of year (in shares)		164,000	146,605	148,535
Shares granted (in shares)		155,100	115,000	100,000
Shares vested (in shares)		(122,010)	(96,375)	(101,930)
Shares forfeited (in shares)		(2,550)	(1,230)	0
Unvested shares, end of year (in shares)		194,540	164,000	146,605
Weighted Average Grant Date Fair Value
Unvested shares, beginning of year (in dollars per share)		$ 28.39	$ 27.93	$ 29.40
Shares granted (in dollars per share)		25.00	28.80	28.39
Shares vested (in dollars per share)		25.09	28.73	28.52
Shares forfeited (in dollars per share)		27.49	0	0
Unvested shares, end of year (in dollars per share)		$ 26.59	$ 28.39	$ 27.93
Unvested shares, scheduled to vest
2020 (in shares)		67,290
2021 (in shares)		55,930
2022 (in shares)		43,600
2023 (in shares)		27,720
Estimated future compensation expense		$ 4,652
Weighted average period over which the compensation expense will be recorded		23 months
Compensation expense		$ 2,849	$ 3,187	$ 2,759
Common Shares | Trustees
Number of Shares
Shares granted (in shares)	3,000	3,000	3,000	3,000