Consolidated Statements of Shareholders’ Equity (unaudited) - USD ($) $ in Thousands	Total	Cumulative effect of change in accounting for credit losses	Common Stock	Capital Surplus	Retained Earnings	Retained Earnings Cumulative effect of change in accounting for credit losses	Accumulated Other Comprehensive Income, Net of Tax	Treasury Stock
Balance at beginning of period (in shares) at Dec. 31, 2019			71,499,829					1,995,996
Balance at beginning of period at Dec. 31, 2019	$ 2,469,582	$ (56,704)	$ 71,500	$ 1,907,108	$ 507,950	$ (56,704)	$ 17,995	$ (34,971)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of restricted shares (in shares)			164,476
Issuance of restricted shares	289		$ 164	125
Forfeitures of restricted shares (in shares)			(11,250)
Forfeitures of restricted shares	(170)		$ (11)	(159)
Proceeds from exercise of stock options (in shares)			49,532
Proceeds from exercise of stock options	981		$ 50	931
Share-based compensation	3,026			3,026
Purchase of treasury shares (in shares)								216,045
Purchase of treasury shares	(7,989)							$ (7,989)
Net income	167,703				167,703
Dividends on common stock shares	(31,292)				(31,292)
Other comprehensive income (loss) during the period	19,257						19,257
Balance at end of period (in shares) at Sep. 30, 2020			71,702,587					2,212,041
Balance at end of period at Sep. 30, 2020	2,564,683		$ 71,703	1,911,031	587,657		37,252	$ (42,960)
Balance at beginning of period (in shares) at Jun. 30, 2020			71,674,087					2,211,305
Balance at beginning of period at Jun. 30, 2020	2,460,130		$ 71,674	1,909,839	481,948		39,613	$ (42,944)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of restricted shares (in shares)			12,500
Issuance of restricted shares	0		$ 13	(13)
Proceeds from exercise of stock options (in shares)			16,000
Proceeds from exercise of stock options	280		$ 16	264
Share-based compensation	941			941
Purchase of treasury shares (in shares)								736
Purchase of treasury shares	(16)							$ (16)
Net income	116,145				116,145
Dividends on common stock shares	(10,436)				(10,436)
Other comprehensive income (loss) during the period	(2,361)						(2,361)
Balance at end of period (in shares) at Sep. 30, 2020			71,702,587					2,212,041
Balance at end of period at Sep. 30, 2020	2,564,683		$ 71,703	1,911,031	587,657		37,252	$ (42,960)
Balance at beginning of period (in shares) at Dec. 31, 2020			71,753,705					(2,212,224)
Balance at beginning of period at Dec. 31, 2020	2,647,088		$ 71,754	1,913,285	671,510		33,505	$ (42,966)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of restricted shares (in shares)			99,308
Issuance of restricted shares	599		$ 99	500
Forfeitures of restricted shares (in shares)			(2,695)
Forfeitures of restricted shares	(53)		$ (3)	(50)
Proceeds from exercise of stock options (in shares)			165,608
Proceeds from exercise of stock options	4,499		$ 166	4,333
Share-based compensation	4,896			4,896
Purchase of treasury shares (in shares)								168,267
Purchase of treasury shares	(8,108)							$ (8,108)
Net income	294,969				294,969
Dividends on common stock shares	(31,500)				(31,500)
Other comprehensive income (loss) during the period	(11,620)						(11,620)
Balance at end of period (in shares) at Sep. 30, 2021			72,015,926					(2,380,491)
Balance at end of period at Sep. 30, 2021	2,900,770		$ 72,016	1,922,964	934,979		21,885	$ (51,074)
Balance at beginning of period (in shares) at Jun. 30, 2021			72,007,871					(2,240,662)
Balance at beginning of period at Jun. 30, 2021	2,837,004		$ 72,008	1,920,566	863,828		25,024	$ (44,422)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Forfeitures of restricted shares (in shares)			(1,945)
Forfeitures of restricted shares	(33)		$ (2)	(31)
Proceeds from exercise of stock options (in shares)			10,000
Proceeds from exercise of stock options	288		$ 10	278
Share-based compensation	2,151			2,151
Purchase of treasury shares (in shares)								139,829
Purchase of treasury shares	(6,652)							$ (6,652)
Net income	81,680				81,680
Dividends on common stock shares	(10,529)				(10,529)
Other comprehensive income (loss) during the period	(3,139)						(3,139)
Balance at end of period (in shares) at Sep. 30, 2021			72,015,926					(2,380,491)
Balance at end of period at Sep. 30, 2021	$ 2,900,770		$ 72,016	$ 1,922,964	$ 934,979		$ 21,885	$ (51,074)