Condensed Consolidated Statements of Changes in Shareholders' Equity - USD ($) $ in Thousands	Common Stock	Surplus	Retained Earnings	Accumulated Other Comprehensive Income (Loss)	Total
Balance at Dec. 31, 2017	$ 91,899	$ 1,807,601	$ 419,847	$ (10,427)	$ 2,308,920
Balance (in shares) at Dec. 31, 2017	36,759,656
Increase (Decrease) in Stockholders' Equity
Comprehensive income			82,785	(22,697)	60,088
Cash dividends declared on common stock per share			(24,651)		(24,651)
AOCI reclassification to retained earnings from adoption of ASU 2018-02			2,947	(2,947)
Employee stock purchases	$ 11	341			352
Employee stock purchases (in shares)	4,338
Stock options exercised	$ 83	948			1,031
Stock options exercised (in shares)	33,424
Restricted stock awards	$ 20	(20)
Restricted stock awards (in shares)	7,836
Stock issued pursuant to restricted stock units.	$ 99	(99)
Stock issued pursuant to restricted stock units (in shares)	39,541
Common stock repurchased	$ (48)	(1,678)			(1,726)
Common stock repurchased (in shares)	(19,239)
Share-based compensation expense		4,353			4,353
Balance at Jun. 30, 2018	$ 92,064	1,811,446	480,928	(36,071)	$ 2,348,367
Balance (in shares) at Jun. 30, 2018	36,825,556				36,825,556
Balance at Dec. 31, 2018	$ 89,574	1,750,495	551,108	(24,881)	$ 2,366,296
Balance (in shares) at Dec. 31, 2018	35,829,549				35,829,549
Increase (Decrease) in Stockholders' Equity
Comprehensive income			85,827	26,347	$ 112,174
Cash dividends declared on common stock per share			(27,491)		(27,491)
Employee stock purchases	$ 15	328			343
Employee stock purchases (in shares)	5,950
Stock options exercised	$ 32	370			402
Stock options exercised (in shares)	12,722
Restricted stock awards	$ 17	(17)
Restricted stock awards (in shares)	6,602
Stock issued pursuant to restricted stock units.	$ 127	(127)
Stock issued pursuant to restricted stock units (in shares)	50,965
Common stock repurchased - buyback plan	$ (2,853)	(77,287)			(80,140)
Common stock repurchased - buyback plan (in shares)	(1,141,200)
Common stock repurchased	$ (73)	(1,890)			(1,963)
Common stock repurchased (in shares)	(29,001)
Share-based compensation expense		4,357			4,357
Balance at Jun. 30, 2019	$ 86,839	$ 1,676,229	$ 609,444	$ 1,466	$ 2,373,978
Balance (in shares) at Jun. 30, 2019	34,735,587				34,735,587