Condensed Consolidated Statements of Changes in Shareholders' Equity - USD ($) $ in Thousands	Common Stock	Surplus	Retained Earnings Cumulative Effect of Adoption of ASU	Retained Earnings	Accumulated Other Comprehensive Income (Loss)	Cumulative Effect of Adoption of ASU	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				137,392			137,392
Other comprehensive income, net of tax effects					27,751		27,751
Total comprehensive income (loss)							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 (forfeitures)	$ 17	(17)
Restricted stock awards (forfeitures)(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, net of tax effects					1,404		1,404
Total comprehensive income (loss)							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
Balance (ASU 2016-13) at Dec. 31, 2019			$ (44,820)			$ (44,820)
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				34,396			$ 34,396
Other comprehensive income, net of tax effects					16,029		16,029
Total comprehensive income (loss)							50,425
Cash dividends declared on common stock per share				(64,907)			(64,907)
Employee stock purchases	$ 40	767					807
Employee stock purchases (in shares)	16,217
Stock options exercised	$ 58	682					740
Stock options exercised (in shares)	23,012
Restricted stock awards (forfeitures)	$ 22	(22)
Restricted stock awards (forfeitures)(in shares)	8,828
Stock issued pursuant to restricted stock units	$ 766	(766)
Stock issued pursuant to restricted stock units (in shares)	306,282
Common stock repurchased - buyback plan	$ (800)	(23,915)					(24,715)
Common stock repurchased - buyback plan (in shares)	(320,000)
Common stock issued for acquisition	$ 93,178	2,153,149					2,246,327
Common stock issued for acquisition (in shares)	37,271,069
Common stock repurchased	$ (304)	(7,334)					(7,638)
Common stock repurchased (in shares)	(121,489)
Share-based compensation expense		18,694					18,694
Stock options and restricted stock acquired and converted pursuant to CSB acquisition		15,487					15,487
Balance at Sep. 30, 2020	$ 177,321	3,764,482		604,564	17,046		$ 4,563,413
Balance (in shares) at Sep. 30, 2020	70,928,304						70,928,304
Balance at Jun. 30, 2020	$ 177,268	3,759,166		542,677	12,739		$ 4,491,850
Balance (in shares) at Jun. 30, 2020	70,907,119
Increase (Decrease) in Stockholders' Equity
Net income				95,221			95,221
Other comprehensive income, net of tax effects					4,307		4,307
Total comprehensive income (loss)							99,528
Cash dividends declared on common stock per share				(33,334)			(33,334)
Employee stock purchases	$ 23	399					422
Employee stock purchases (in shares)	9,316
Stock options exercised	$ 14	166					180
Stock options exercised (in shares)	5,471
Stock issued pursuant to restricted stock units	$ 26	(26)
Stock issued pursuant to restricted stock units (in shares)	10,207
Common stock repurchased	$ (10)	(183)					(193)
Common stock repurchased (in shares)	(3,809)
Share-based compensation expense		4,960					4,960
Balance at Sep. 30, 2020	$ 177,321	$ 3,764,482		$ 604,564	$ 17,046		$ 4,563,413
Balance (in shares) at Sep. 30, 2020	70,928,304						70,928,304