Condensed Consolidated Statements of Changes in Shareholders' Equity (Unaudited) - USD ($) $ in Thousands	Total	Common Shares	Preferred Shares	Additional Paid-in Capital	Retained Earnings	Accumulated Other Comprehensive Income (Loss)
Balance (in shares) at Dec. 31, 2017		29,696,682
Balance at Dec. 31, 2017	$ 694,699	$ 6	$ 0	$ 636,149	$ 48,198	$ 10,346
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	32,617				32,617
Other comprehensive income (loss)	(25,105)					(25,105)
Dividends	(18,109)				(18,109)
Exercise of stock options (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
Balance at Jun. 30, 2018	689,243	$ 6	0	641,290	66,677	(18,730)
Balance (in shares) at Mar. 31, 2018		29,866,705
Balance at Mar. 31, 2018	685,770	$ 6	0	639,183	58,753	(12,172)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	16,984				16,984
Other comprehensive income (loss)	(6,558)					(6,558)
Dividends	(9,060)				(9,060)
Exercise of stock options (in shares)		50,639
Exercise of stock options	448			448
Vesting of RSUs (in shares)		477
Vesting of RSUs	(1)			(1)
Compensation expense under share incentive plans	1,660			1,660
Balance (in shares) at Jun. 30, 2018		29,917,821
Balance at Jun. 30, 2018	$ 689,243	$ 6	0	641,290	66,677	(18,730)
Balance (in shares) at Dec. 31, 2018	29,988,460	29,988,460
Balance at Dec. 31, 2018	$ 709,241	$ 6	0	645,310	79,753	(15,828)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	43,035				43,035
Other comprehensive income (loss)	40,992					40,992
Dividends	(18,339)				(18,339)
Exercise of stock options (in shares)		265,938
Exercise of stock options	5,731			5,731
Vesting of RSUs (in shares)		76,277
Vesting of RSUs	(1,374)			(1,374)
Compensation expense under share incentive plans	$ 3,484			3,484
Balance (in shares) at Jun. 30, 2019	30,330,675	30,330,675
Balance at Jun. 30, 2019	$ 791,050	$ 6	0	653,151	112,729	25,164
Balance (in shares) at Mar. 31, 2019		30,162,045
Balance at Mar. 31, 2019	754,297	$ 6	0	648,242	101,617	4,432
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	20,307				20,307
Other comprehensive income (loss)	20,732					20,732
Dividends	(9,195)				(9,195)
Exercise of stock options (in shares)		166,963
Exercise of stock options	3,099			3,099
Vesting of RSUs (in shares)		1,667
Vesting of RSUs	0			0
Compensation expense under share incentive plans	$ 1,810			1,810
Balance (in shares) at Jun. 30, 2019	30,330,675	30,330,675
Balance at Jun. 30, 2019	$ 791,050	$ 6	$ 0	$ 653,151	$ 112,729	$ 25,164