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, 2015		28,941,547
Balance at Dec. 31, 2015	$ 681,038	$ 6	$ 630,820	$ 47,026	$ 3,186
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	27,433			27,433
Other comprehensive income	28,412				28,412
Dividends	(11,722)			(11,722)
Exercised (in shares)		149,949
Exercise of stock options and related excess tax benefits	2,050		2,050
Compensation expense under stock incentive plans	2,687		2,687
Balance (in shares) at Jun. 30, 2016		29,091,496
Balance at Jun. 30, 2016	$ 729,898	$ 6	635,557	62,737	31,598
Balance (in shares) at Dec. 31, 2016	29,257,566	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)
Compensation expense under stock incentive plans	3,578		3,578
Vesting of restricted share units (in shares)		24,922
Vesting of restricted share units	$ (627)		(627)
Balance (in shares) at Jun. 30, 2017	29,467,647	29,467,647
Balance at Jun. 30, 2017	$ 719,719	$ 6	$ 637,349	$ 70,495	$ 11,869