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	32,991			32,991
Other comprehensive income	10,742				10,742
Dividends	(17,728)			(17,728)
Exercised (in shares)		185,159
Exercise of stock options	(2,458)		(2,458)
Vesting of RSUs (in shares)		24,922
Vesting of RSUs	(627)		(627)
Compensation expense under share incentive plans	3,578		3,578
Balance (in shares) at Jun. 30, 2017		29,467,647
Balance at Jun. 30, 2017	$ 719,719	$ 6	637,349	70,495	11,869
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	32,617			32,617
Net income | ASU 2016-01				(922)
Other comprehensive income	(25,105)				(25,105)
Dividends	(18,109)			(18,109)
Exercised (in shares)		177,835
Exercise of stock options	2,803		2,803
Vesting of RSUs (in shares)		43,304
Vesting of RSUs	(777)		(777)
Compensation expense under share incentive plans	$ 3,115		3,115
Balance (in shares) at Jun. 30, 2018	29,917,821	29,917,821
Balance at Jun. 30, 2018	$ 689,243	$ 6	$ 641,290	66,677	(18,730)
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