Shareholders' Equity - Share-based Compensation (Details) - Award Plans - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Number of Shares
Unvested shares, beginning of year (in shares)	148,535	147,004	150,192
Shares granted (in shares)	100,000	91,600	91,250
Shares vested (in shares)	(101,930)	(90,069)	(93,248)
Shares forfeited (in shares)	0	0	(1,190)
Unvested shares, end of year (in shares)	146,605	148,535	147,004
Weighted Average Grant Date Fair Value
Unvested shares, beginning of year (in dollars per share)	$ 29.40	$ 25.95	$ 28.52
Shares granted (in dollars per share)	28.39	28.15	26.32
Shares vested (in dollars per share)	28.52	28.07	27.16
Shares forfeited (in dollars per share)	0.00	0.00	0.00
Unvested shares, end of year (in dollars per share)	$ 27.93	$ 29.40	$ 25.95
Unvested shares, scheduled to vest
2018 (in shares)	56,475
2019 (in shares)	43,750
2020 (in shares)	30,480
2021 (in shares)	15,900
Estimated future compensation expense	$ 4,376
Closing price of common shares (in dollars per share)	$ 29.85
Weighted average period over which the compensation expense will be recorded	21 months
Compensation expense	$ 2,759	$ 2,834	$ 2,475