Condensed Consolidated Statements of Changes in Shareholders' Equity (Unaudited) - USD ($) $ in Thousands	Total	Common Shares	Additional Paid-in Capital	Retained Earnings	Accumulated Other Comprehensive Income
Balance (in shares) at Dec. 31, 2016		29,257,566
Balance at Dec. 31, 2016	$ 693,221	$ 6	$ 636,856	$ 55,232	$ 1,127
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	18,450			18,450
Other comprehensive income	4,034				4,034
Dividends	(8,810)			(8,810)
Exercised (in shares)		63,467
Exercise of stock options	198		198
Vesting of RSUs (in shares)		23,294
Vesting of RSUs	(627)		(627)
Compensation expense under share incentive plans	1,794		1,794
Balance (in shares) at Mar. 31, 2017		29,344,327
Balance at Mar. 31, 2017	$ 708,260	$ 6	638,221	64,872	5,161
Balance (in shares) at Dec. 31, 2017	29,696,682	29,696,682
Balance at Dec. 31, 2017	$ 694,699	$ 6	636,149	48,198	10,346
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	15,633			15,633
Net income | ASU 2016-01				(1,400)
Other comprehensive income	(18,547)				(18,547)
Dividends	(9,049)			(9,049)
Exercised (in shares)		127,196
Exercise of stock options	2,355		2,355
Vesting of RSUs (in shares)		42,827
Vesting of RSUs	(776)		(776)
Compensation expense under share incentive plans	$ 1,455		1,455
Balance (in shares) at Mar. 31, 2018	29,866,705	29,866,705
Balance at Mar. 31, 2018	$ 685,770	$ 6	$ 639,183	58,753	(12,172)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Cumulative effect of adoption of ASU | ASU 2016-01	0			4,682	(4,682)
Cumulative effect of adoption of ASU | ASU 2018-02	$ 0			$ (711)	$ 711