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			51,906			51,906
Dividends:
Dividends			(32,822)			(32,822)
Purchase of treasury stock					(54)	(54)
Exercise of stock options	$ 33	565				598
Exercise of stock options (in shares)	33
Stock compensation expense recognized in earnings		265				265
Other comprehensive income, net of tax:
Net change in unrealized gains and losses on available for sale securities, net of reclassification adjustments				25,229		25,229
Balance at Mar. 31, 2019	$ 96,137	146,113	2,083,218	(29,405)	(311,359)	1,984,704
Balance (in shares) at Mar. 31, 2019	96,137
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			34,838			34,838
Dividends:
Dividends			(35,128)			(35,128)
Purchase of treasury stock					(29,371)	(29,371)
Exercise of stock options	$ 11	214				225
Exercise of stock options (in shares)	11
Stock compensation expense recognized in earnings		219				219
Other comprehensive income, net of tax:
Net change in unrealized gains and losses on available for sale securities, net of reclassification adjustments				62,007		62,007
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
Dividends:
Cumulative adjustment for adoption of new accounting standards, net of tax			$ (8,332)			$ (8,332)