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
Net income			129,867		129,867
Other comprehensive income (loss), net of tax effects				(31,135)	(31,135)
Comprehensive income					98,732
Cash dividends declared on common stock per share			(37,506)		(37,506)
AOCI reclassification to retained earnings from adoption of ASU 2018-02			2,947	(2,947)
Employee stock purchases	$ 22	687			709
Employee stock purchases (in shares)	8,783
Stock options exercised	$ 83	948			1,031
Stock options exercised (in shares)	33,424
Restricted stock awards	$ 12	(12)
Restricted stock awards (in shares)	4,586
Stock issued pursuant to restricted stock units.	$ 99	(99)
Stock issued pursuant to restricted stock units (in shares)	39,541
Common stock repurchased - buyback plan	$ (250)	(8,094)			(8,344)
Common stock repurchased - buyback plan (in shares)	(100,000)
Common stock repurchased	$ (57)	(1,959)			(2,016)
Common stock repurchased (in shares)	(22,752)
Share-based compensation expense		6,613			6,613
Balance at Sep. 30, 2018	$ 91,808	1,805,685	515,155	(44,509)	$ 2,368,139
Balance (in shares) at Sep. 30, 2018	36,723,238				36,723,238
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
Increase (Decrease) in Stockholders' Equity
Net income			47,082		47,082
Other comprehensive income (loss), net of tax effects				(8,438)	(8,438)
Comprehensive income					38,644
Cash dividends declared on common stock per share			(12,855)		(12,855)
Employee stock purchases	$ 11	346			357
Employee stock purchases (in shares)	4,445
Restricted stock awards	$ (8)	8
Restricted stock awards (in shares)	(3,250)
Common stock repurchased - buyback plan	$ (250)	(8,094)			(8,344)
Common stock repurchased - buyback plan (in shares)	(100,000)
Common stock repurchased	$ (9)	(281)			(290)
Common stock repurchased (in shares)	(3,513)
Share-based compensation expense		2,260			2,260
Balance at Sep. 30, 2018	$ 91,808	1,805,685	515,155	(44,509)	$ 2,368,139
Balance (in shares) at Sep. 30, 2018	36,723,238				36,723,238
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
Net income			137,392		$ 137,392
Other comprehensive income (loss), net of tax effects				27,751	27,751
Comprehensive income					165,143
Cash dividends declared on common stock per share			(42,175)		(42,175)
Employee stock purchases	$ 29	688			717
Employee stock purchases (in shares)	11,548
Stock options exercised	$ 93	1,137			1,230
Stock options exercised (in shares)	36,978
Restricted stock awards	$ 17	(17)
Restricted stock awards (in shares)	6,602
Stock issued pursuant to restricted stock units.	$ 128	(128)
Stock issued pursuant to restricted stock units (in shares)	51,543
Common stock repurchased - buyback plan	$ (5,000)	(139,594)			(144,594)
Common stock repurchased - buyback plan (in shares)	(2,000,000)
Common stock repurchased	$ (84)	(2,219)			(2,303)
Common stock repurchased (in shares)	(33,494)
Share-based compensation expense		6,642			6,642
Balance at Sep. 30, 2019	$ 84,757	1,617,004	646,325	2,870	$ 2,350,956
Balance (in shares) at Sep. 30, 2019	33,902,726				33,902,726
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
Increase (Decrease) in Stockholders' Equity
Net income			51,565		51,565
Other comprehensive income (loss), net of tax effects				1,404	1,404
Comprehensive income					52,969
Cash dividends declared on common stock per share			(14,684)		(14,684)
Employee stock purchases	$ 14	360			374
Employee stock purchases (in shares)	5,598
Stock options exercised	$ 61	767			828
Stock options exercised (in shares)	24,256
Stock issued pursuant to restricted stock units.	$ 1	(1)
Stock issued pursuant to restricted stock units (in shares)	578
Common stock repurchased - buyback plan	$ (2,147)	(62,307)			(64,454)
Common stock repurchased - buyback plan (in shares)	(858,800)
Common stock repurchased	$ (11)	(329)			(340)
Common stock repurchased (in shares)	(4,493)
Share-based compensation expense		2,285			2,285
Balance at Sep. 30, 2019	$ 84,757	$ 1,617,004	$ 646,325	$ 2,870	$ 2,350,956
Balance (in shares) at Sep. 30, 2019	33,902,726				33,902,726