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	48,799			48,799
Other comprehensive income	26,740				26,740
Dividends	(17,604)			(17,604)
Exercised (in shares)		174,949
Exercise of stock options and related excess tax benefits	2,585		2,585
Compensation expense under stock incentive plans	4,207		4,207
Balance (in shares) at Sep. 30, 2016		29,116,496
Balance at Sep. 30, 2016	$ 745,765	$ 6	637,612	78,221	29,926
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	43,342			43,342
Other comprehensive income	11,458				11,458
Dividends	(26,670)			(26,670)
Exercised (in shares)		300,168
Exercise of stock options	(5,093)		(5,093)
Compensation expense under stock incentive plans	5,338		5,338
Vesting of restricted share units (in shares)		24,922
Vesting of restricted share units	$ (627)		(627)
Balance (in shares) at Sep. 30, 2017	29,582,656	29,582,656
Balance at Sep. 30, 2017	$ 720,969	$ 6	$ 636,474	$ 71,904	$ 12,585