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, 2018	$ 89,574	$ 1,750,495	$ 551,108	$ (24,881)	$ 2,366,296
Balance (in shares) at Dec. 31, 2018	35,829,549
Increase (Decrease) in Stockholders' Equity
Net income			44,367		44,367
Other comprehensive income (loss), net of tax effects				11,507	11,507
Comprehensive income					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
Balance at Dec. 31, 2019	$ 84,361	1,607,740	679,895	1,017	$ 2,373,013
Balance (in shares) at Dec. 31, 2019	33,744,385				33,744,385
Increase (Decrease) in Stockholders' Equity
Net income			24,110		$ 24,110
Other comprehensive income (loss), net of tax effects				8,748	8,748
Comprehensive income					32,858
Cash dividends declared on common stock per share			(15,840)		(15,840)
Stock options exercised	$ 31	372			403
Stock options exercised (in shares)	12,485
Restricted stock awards	$ 1	(1)
Restricted stock awards (in shares)	498
Stock issued pursuant to restricted stock units	$ 79	(79)
Stock issued pursuant to restricted stock units (in shares)	31,435
Common stock repurchased - buyback plan	$ (800)	(23,914)			(24,714)
Common stock repurchased - buyback plan (in shares)	(320,000)
Common stock repurchased	$ (61)	(1,803)			(1,864)
Common stock repurchased (in shares)	(24,567)
Share-based compensation expense		2,007			2,007
Balance at Mar. 31, 2020	$ 83,611	$ 1,584,322	643,345	$ 9,765	$ 2,321,043
Balance (in shares) at Mar. 31, 2020	33,444,236				33,444,236
Increase (Decrease) in Stockholders' Equity
Cumulative change in accounting principle due to the adoption of ASU 2016-13 | ASU 2016-13			$ (44,820)		$ (44,820)