Condensed Consolidated Statements of Changes in Shareholders' Equity - USD ($) $ in Thousands	Total		Common Stock [Member]	Additional Paid-in Capital [Member]	Retained Earnings [Member]		Accumulated Other Comprehensive (Loss) Income [Member]		Treasury Stock [Member]
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Cumulative effect of new accounting principle in period of adoption | Accounting Standards Update 2016-01	$ 0		$ 0	$ 0	$ 433		$ (433)		$ 0
Cumulative effect of new accounting principle in period of adoption | Accounting Standards Update 2018-02	0		0	0	3,921		(3,921)		0
Beginning balance at Dec. 31, 2017	603,374		$ 157,784	290,133	216,761		(17,771)		(43,533)
Beginning balance, shares at Dec. 31, 2017			29,334,859
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	32,171		$ 0	0	32,171		0		0
Other comprehensive income, net of income tax	(6,539)		0	0	0		(6,539)		0
Cash dividends declared	(17,629)		0	0	(17,629)		0		0
Stock issued under dividend reinvestment and employee stock purchase plans	1,739		$ 0	140	1		0		1,598
Stock issued under dividend reinvestment and employee stock purchase plans, shares			62,271
Exercise of stock options	1,131		$ 0	(43)	0		0		1,174
Exercise of stock options, shares			59,750
Stock-based compensation	2,379		$ 0	2,379	0		0		0
Purchases of treasury stock	(2,384)		$ 0	0	0		0		(2,384)
Purchases of treasury stock, shares			(83,977)
Restricted stock awards granted, net of cancellations	0		$ 0	(617)	0		0		617
Restricted stock awards granted, net of cancellations, shares			34,173
Ending balance at Sep. 30, 2018	$ 614,242		$ 157,784	291,992	235,658		(28,664)		(42,528)
Ending balance, shares at Sep. 30, 2018			29,407,076
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Cash dividends declared, per share	$ 0.60
Beginning balance at Jun. 30, 2018	$ 605,294		$ 157,784	291,238	226,574		(28,007)		(42,295)
Beginning balance, shares at Jun. 30, 2018			29,406,450
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	14,964		$ 0	0	14,964		0		0
Other comprehensive income, net of income tax	(657)		0	0	0		(657)		0
Cash dividends declared	(5,880)		0	0	(5,880)		0		0
Stock issued under dividend reinvestment and employee stock purchase plans	566		$ 0	42	0		0		524
Stock issued under dividend reinvestment and employee stock purchase plans, shares			20,332
Exercise of stock options	420		$ 0	(29)	0		0		449
Exercise of stock options, shares			22,760
Stock-based compensation	660		$ 0	660	0		0		0
Purchases of treasury stock	(1,125)		$ 0	0	0		0		(1,125)
Purchases of treasury stock, shares			(39,624)
Restricted stock awards granted, net of cancellations	0		$ 0	81	0		0		(81)
Restricted stock awards granted, net of cancellations, shares			(2,842)
Ending balance at Sep. 30, 2018	$ 614,242		$ 157,784	291,992	235,658		(28,664)		(42,528)
Ending balance, shares at Sep. 30, 2018			29,407,076
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Cash dividends declared, per share	$ 0.20
Cumulative effect of new accounting principle in period of adoption | Accounting Standards Update 2016-02	$ (1,525)	[1]	$ 0	0	(1,525)	[1]	0		0
Cumulative effect of new accounting principle in period of adoption | Accounting Standards Update 2017-12	0		0	0	(83)	[1]	83	[1]	0
Cumulative effect of new accounting principle in period of adoption | Accounting Standards Update 2017-08	(39)	[1]	0	0	(39)	[1]	0		0
Beginning balance at Dec. 31, 2018	624,133		$ 157,784	292,401	248,167		(28,416)		(45,803)
Beginning balance, shares at Dec. 31, 2018			29,270,852
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	50,209		$ 0	0	50,209		0		0
Other comprehensive income, net of income tax	6,325		0	0	0		6,325		0
Cash dividends declared	(17,572)		0	0	(17,572)		0		0
Stock issued under dividend reinvestment and employee stock purchase plans	1,669		$ 0	114	1		0		1,554
Stock issued under dividend reinvestment and employee stock purchase plans, shares			69,126
Exercise of stock options	$ 1,005		$ 0	(170)	0		0		1,175
Exercise of stock options, shares	58,545		58,545
Stock-based compensation	$ 1,870		$ 0	1,870	0		0		0
Purchases of treasury stock	(1,776)		$ 0	0	0		0		(1,776)
Purchases of treasury stock, shares			(68,640)
Cancellations of performance-based restricted stock awards	0		$ 0	341	0		0		(341)
Cancellation of performance-based restricted stock awards, shares			(17,349)
Ending balance at Sep. 30, 2019	$ 664,299		$ 157,784	294,556	279,158		(22,008)		(45,191)
Ending balance, shares at Sep. 30, 2019			29,312,534
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Cash dividends declared, per share	$ 0.60
Beginning balance at Jun. 30, 2019	$ 651,670		$ 157,784	293,947	267,357		(21,949)		(45,469)
Beginning balance, shares at Jun. 30, 2019			29,294,942
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	17,662		$ 0	0	17,662		0		0
Other comprehensive income, net of income tax	(59)		0	0	0		(59)		0
Cash dividends declared	(5,861)		0	0	(5,861)		0		0
Stock issued under dividend reinvestment and employee stock purchase plans	534		$ 0	37	0		0		497
Stock issued under dividend reinvestment and employee stock purchase plans, shares			20,943
Exercise of stock options	314		$ 0	(68)	0		0		382
Exercise of stock options, shares			19,045
Stock-based compensation	640		$ 0	640	0		0		0
Purchases of treasury stock	(601)		$ 0	0	0		0		(601)
Purchases of treasury stock, shares			(22,396)
Ending balance at Sep. 30, 2019	$ 664,299		$ 157,784	$ 294,556	$ 279,158		$ (22,008)		$ (45,191)
Ending balance, shares at Sep. 30, 2019			29,312,534
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Cash dividends declared, per share	$ 0.20

[1] (1) See Note 1, "Summary of Significant Accounting Policies - Accounting Pronouncements Adopted in 2019" for additional information.