Consolidated Statement of Changes in Shareholders' Equity - USD ($) $ in Thousands	Total	Common Stock [Member]	Surplus [Member]	Retained Earnings [Member]	Accumulated Other Comprehensive Income (Loss) [Member]	Treasury Stock [Member]
Beginning Balance at Dec. 31, 2018	$ 3,251,624	$ 263,098	$ 2,134,462	$ 1,013,037	$ (57,019)	$ (101,954)
Beginning Balance, shares at Dec. 31, 2018		105,239,121
Cumulative effect of adopting Accounting Standard Update	(1,049)	$ 0	0	(1,049)	0	0
Reclass due to adopting Accounting Standard Update	50	0	0	0	50	0
Net income	63,642	0	0	63,642	0	0
Other comprehensive income, net of tax	17,699	0	0	0	17,699	0
Comprehensive income, net of tax	81,341
Stock based compensation expense	1,113	0	1,113	0	0	0
Purchase of treasury stock	(12,072)	0	0	0	0	(12,072)
Cash dividends	(34,759)			(34,759)	0	0
Grant of restricted stock	$ 0	316	(316)	0	0	0
Grant of restricted stock, shares	126,427
Common stock options exercised	$ 643	$ 84	559	0	0	0
Common stock options exercised, shares	33,816	33,816
Ending Balance at Mar. 31, 2019	$ 3,286,891	$ 263,498	2,135,818	1,040,871	(39,270)	(114,026)
Ending Balance, shares at Mar. 31, 2019		105,399,364
Beginning Balance at Dec. 31, 2019	3,363,833	$ 263,736	2,140,175	1,132,579	(34,869)	(137,788)
Beginning Balance, shares at Dec. 31, 2019		105,494,290
Cumulative effect of adopting Accounting Standard Update	(44,331)	$ 0	0	(44,331)	0	0
Net income	40,183	0	0	40,183	0	0
Other comprehensive income, net of tax	18,698	0	0	0	18,698	0
Comprehensive income, net of tax	58,881
Stock based compensation expense	1,253	0	1,253	0	0	0
Purchase of treasury stock	(608)	0	0	0	0	(608)
Cash dividends	(35,604)	0	0	(35,604)	0	0
Grant of restricted stock	$ 0	439	(439)	0	0	0
Grant of restricted stock, shares	175,495
Forfeiture of restricted stock	$ 0	0	35	0	0	(35)
Common stock options exercised	$ 278	$ 36	242	0	0	0
Common stock options exercised, shares	14,694	14,694
Ending Balance at Mar. 31, 2020	$ 3,343,702	$ 264,211	$ 2,141,266	$ 1,092,827	$ (16,171)	$ (138,431)
Ending Balance, shares at Mar. 31, 2020		105,684,479