Consolidated Statements of Shareholders' Equity - USD ($) shares in Thousands, $ in Thousands	Common Stock	Surplus	Retained Earnings	Other Comprehensive Income (Loss)	Treasury Stock	Total
Balance at Dec. 31, 2018	$ 96,104	$ 145,283	$ 2,064,134	$ (54,634)	$ (311,305)	$ 1,939,582
Balance (in shares) at Dec. 31, 2018	96,104
Increase (decrease) in shareholders' equity
Net Income			101,926			101,926
Dividends:
Cash			(32,821)			(32,821)
Purchase of treasury stock					(128)	(128)
Exercise of stock options	$ 47	827				874
Exercise of stock options (in shares)	47
Stock compensation expense recognized in earnings		504				504
Other comprehensive income, net of tax:
Net change in unrealized gains and losses on available for sale securities, net of reclassification adjustments				50,382		50,382
Balance at Jun. 30, 2019	$ 96,151	146,614	2,133,239	(4,252)	(311,433)	2,060,319
Balance (in shares) at Jun. 30, 2019	96,151
Balance at Mar. 31, 2019	$ 96,137	146,113	2,083,219	(29,405)	(311,359)	1,984,705
Balance (in shares) at Mar. 31, 2019	96,137
Increase (decrease) in shareholders' equity
Net Income			50,020			50,020
Dividends:
Purchase of treasury stock					(74)	(74)
Exercise of stock options	$ 14	262				276
Exercise of stock options (in shares)	14
Stock compensation expense recognized in earnings		239				239
Other comprehensive income, net of tax:
Net change in unrealized gains and losses on available for sale securities, net of reclassification adjustments				25,153		25,153
Balance at Jun. 30, 2019	$ 96,151	146,614	2,133,239	(4,252)	(311,433)	2,060,319
Balance (in shares) at Jun. 30, 2019	96,151
Balance at Dec. 31, 2019	$ 96,215	148,075	2,200,568	2,345	(329,150)	2,118,053
Balance (in shares) at Dec. 31, 2019	96,215
Increase (decrease) in shareholders' equity
Net Income			76,348			76,348
Dividends:
Cash			(35,128)			(35,128)
Purchase of treasury stock					(48,737)	(48,737)
Exercise of stock options	$ 14	274				288
Exercise of stock options (in shares)	14
Stock compensation expense recognized in earnings		395				395
Other comprehensive income, net of tax:
Net change in unrealized gains and losses on available for sale securities, net of reclassification adjustments				38,974		38,974
Balance at Jun. 30, 2020	$ 96,229	148,744	2,233,455	41,319	(377,887)	2,141,860
Balance (in shares) at Jun. 30, 2020	96,229
Balance at Mar. 31, 2020	$ 96,226	148,508	2,191,946	64,352	(358,521)	2,142,511
Balance (in shares) at Mar. 31, 2020	96,226
Increase (decrease) in shareholders' equity
Net Income			41,509			41,509
Dividends:
Purchase of treasury stock					(19,366)	(19,366)
Exercise of stock options	$ 3	59				62
Exercise of stock options (in shares)	3
Stock compensation expense recognized in earnings		177				176
Other comprehensive income, net of tax:
Net change in unrealized gains and losses on available for sale securities, net of reclassification adjustments				(23,033)		(23,033)
Balance at Jun. 30, 2020	$ 96,229	$ 148,744	2,233,455	$ 41,319	$ (377,887)	2,141,860
Balance (in shares) at Jun. 30, 2020	96,229
Dividends:
Cumulative adjustment for adoption of new accounting standards, net of tax			$ (8,333)			$ (8,333)