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 (loss) income			85,827		85,827
Other comprehensive income (loss), net of tax effects				26,347	26,347
Total comprehensive income (loss)					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 (forfeitures)	$ 17	(17)
Restricted stock awards (forfeitures)(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
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
Increase (Decrease) in Stockholders' Equity
Net (loss) income			41,460		41,460
Other comprehensive income (loss), net of tax effects				14,840	14,840
Total comprehensive income (loss)					56,300
Cash dividends declared on common stock per share			(14,050)		(14,050)
Employee stock purchases	$ 15	328			343
Employee stock purchases (in shares)	5,950
Restricted stock awards (forfeitures)	$ 16	(16)
Restricted stock awards (forfeitures)(in shares)	6,571
Stock issued pursuant to restricted stock units	$ 5	(5)
Stock issued pursuant to restricted stock units (in shares)	1,736
Common stock repurchased - buyback plan	$ (1,603)	(45,270)			(46,873)
Common stock repurchased - buyback plan (in shares)	(641,200)
Common stock repurchased	$ (15)	(430)			(445)
Common stock repurchased (in shares)	(5,991)
Share-based compensation expense		2,226			2,226
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
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 (loss) income			(60,825)		$ (60,825)
Other comprehensive income (loss), net of tax effects				11,722	11,722
Total comprehensive income (loss)					(49,103)
Cash dividends declared on common stock per share			(31,573)		(31,573)
Employee stock purchases	$ 17	368			385
Employee stock purchases (in shares)	6,901
Stock options exercised	$ 44	516			560
Stock options exercised (in shares)	17,541
Restricted stock awards (forfeitures)	$ 22	(22)
Restricted stock awards (forfeitures)(in shares)	8,828
Stock issued pursuant to restricted stock units	$ 740	(740)
Stock issued pursuant to restricted stock units (in shares)	296,075
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	$ (294)	(7,151)			(7,445)
Common stock repurchased (in shares)	(117,680)
Share-based compensation expense		13,734			13,734
Stock options and restricted stock acquired and converted pursuant to CSB acquisition		15,487			15,487
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				70,907,119
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
Increase (Decrease) in Stockholders' Equity
Net (loss) income			(84,935)		(84,935)
Other comprehensive income (loss), net of tax effects				2,974	2,974
Total comprehensive income (loss)					(81,961)
Cash dividends declared on common stock per share			(15,733)		(15,733)
Employee stock purchases	$ 17	368			385
Employee stock purchases (in shares)	6,901
Stock options exercised	$ 13	143			156
Stock options exercised (in shares)	5,056
Restricted stock awards (forfeitures)	$ 21	(21)
Restricted stock awards (forfeitures)(in shares)	8,330
Stock issued pursuant to restricted stock units	$ 661	(661)
Stock issued pursuant to restricted stock units (in shares)	264,640
Common stock repurchased - buyback plan	$ (233)	(5,348)			(5,581)
Common stock repurchased - buyback plan (in shares)	(93,113)
Common stock issued for acquisition	$ 93,178	2,153,149			2,246,327
Common stock issued for acquisition (in shares)	37,271,069
Share-based compensation expense		11,727			11,727
Stock options and restricted stock acquired and converted pursuant to CSB acquisition		15,487			15,487
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				70,907,119
Increase (Decrease) in Stockholders' Equity
Cumulative change in accounting principle due to the adoption of ASU 2016-13			$ (44,820)		$ (44,820)