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			42,326	(16,976)	25,350
Cash dividends declared on common stock per share			(12,138)		(12,138)
AOCI reclassification to retained earnings from adoption of ASU 2018-02			2,947	(2,947)
Stock options exercised	$ 5	61			66
Stock options exercised (in shares)	2,240
Restricted stock awards	$ 3	(3)
Restricted stock awards (in shares)	1,169
Stock issued pursuant to restricted stock units.	$ 96	(96)
Stock issued pursuant to restricted stock units (in shares)	38,365
Common stock repurchased	$ (45)	(1,567)			(1,612)
Common stock repurchased (in shares)	(17,992)
Share-based compensation expense		1,993			1,993
Balance at Mar. 31, 2018	$ 91,958	1,807,989	452,982	(30,350)	$ 2,322,579
Balance (in shares) at Mar. 31, 2018	36,783,438				36,783,438
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			44,367	11,507	$ 55,874
Cash dividends declared on common stock per share			(13,441)		(13,441)
Stock options exercised	$ 32	370			402
Stock options exercised (in shares)	12,722
Restricted stock awards	$ 1	(1)
Restricted stock awards (in shares)	31
Stock issued pursuant to restricted stock units.	$ 122	(122)
Stock issued pursuant to restricted stock units (in shares)	49,229
Common stock repurchased - buyback plan	$ (1,250)	(32,017)			(33,267)
Common stock repurchased - buyback plan (in shares)	(500,000)
Common stock repurchased	$ (58)	(1,460)			(1,518)
Common stock repurchased (in shares)	(23,010)
Share-based compensation expense		2,131			2,131
Balance at Mar. 31, 2019	$ 88,421	$ 1,719,396	$ 582,034	$ (13,374)	$ 2,376,477
Balance (in shares) at Mar. 31, 2019	35,368,521				35,368,521