Shareholders' Equity - Share-based Compensation (Details) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Number of Shares
Unvested shares, beginning of year (in shares)	147,004	150,192	145,365
Shares granted (in shares)	91,600	91,250	92,225
Shares vested (in shares)	(90,069)	(93,248)	(87,398)
Shares forfeited (in shares)	0	(1,190)	0
Unvested shares, end of year (in shares)	148,535	147,004	150,192
Weighted Average Grant Date Fair Value
Unvested shares, beginning of year (in dollars per share)	$ 25.95	$ 28.52	$ 27.55
Shares granted (in dollars per share)	28.15	26.32	28.61
Shares vested (in dollars per share)	28.07	27.16	28.69
Shares forfeited (in dollars per share)	0.00	0.00	0.00
Unvested shares, end of year (in dollars per share)	$ 29.40	$ 25.95	$ 28.52
Award Plans
Unvested shares, scheduled to vest
2017	58,090
2018	44,295
2019	30,330
2020	15,820
Estimated future compensation expense	$ 4,715
Closing price of common shares (in dollars per share)	$ 31.74
Weighted average period over which the compensation expense will be recorded	22 months
Compensation expense	$ 2,834	$ 2,475	$ 2,437