Consolidated Statements of Changes in Shareholders' Equity - USD ($)	Total	Common Shares	Preferred stock	Additional Paid-in Capital	Retained Earnings	Accumulated Other Comprehensive Income (Loss)
Balances at Dec. 31, 2016	$ 693,221,000	$ 6,000	$ 0	$ 636,856,000	$ 55,232,000	$ 1,127,000
Balances (in shares) at Dec. 31, 2016		29,257,566
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	43,566,000				43,566,000
Other comprehensive income (loss)	9,219,000					9,219,000
Dividends	(50,600,000)				(50,600,000)
Exercise of stock options	(6,213,000)			(6,213,000)
Vesting of RSUs and related excess tax benefits (in shares)		80,149
Vesting of RSUs and related excess tax benefits	(2,182,000)			(2,182,000)
Exercise of stock options (in shares)		358,967
Compensation expense under share incentive plans	7,688,000			7,688,000
Balances at Dec. 31, 2017	$ 694,699,000	$ 6,000	0	636,149,000	48,198,000	10,346,000
Balances (in shares) at Dec. 31, 2017	29,696,682	29,696,682
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	$ 15,633,000
Balances at Dec. 31, 2017	$ 694,699,000	$ 6,000	0	636,149,000	48,198,000	10,346,000
Balances (in shares) at Dec. 31, 2017	29,696,682	29,696,682
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	$ 63,830,000				63,830,000
Other comprehensive income (loss)	(22,203,000)					(22,203,000)
Dividends	(36,246,000)				(36,246,000)
Exercise of stock options	4,045,000			4,045,000
Exercise of stock options (in shares)		237,319
Vesting of RSUs	(1,112,000)			(1,112,000)
Vesting of RSUs (in shares)		54,459
Compensation expense under share incentive plans	6,228,000			6,228,000
Balances at Dec. 31, 2018	$ 709,241,000	$ 6,000	0	645,310,000	79,753,000	(15,828,000)
Balances (in shares) at Dec. 31, 2018	29,988,460	29,988,460
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	$ 38,339,000				38,339,000
Other comprehensive income (loss)	46,942,000					46,942,000
Dividends	(36,786,000)				(36,786,000)
Exercise of stock options	7,124,000			7,124,000
Exercise of stock options (in shares)		345,434
Vesting of RSUs	(1,737,000)			(1,737,000)
Vesting of RSUs (in shares)		90,497
Compensation expense under share incentive plans	7,178,000			7,178,000
Balances at Dec. 31, 2019	$ 778,581,000	$ 6,000	$ 0	$ 657,875,000	$ 89,586,000	$ 31,114,000
Balances (in shares) at Dec. 31, 2019	30,424,391	30,424,391