Consolidated Statements of Shareholders' Equity - USD ($) shares in Thousands, $ in Thousands	Common Stock	Surplus	Retained Earnings	Other Comprehensive Income (Loss)	Treasury Stock	Total
Dividends:
Cumulative adjustment for adoption of new accounting standards			$ 5,997	$ (5,997)
Balance at Dec. 31, 2017	$ 96,019	$ 171,816	1,891,805	(28,397)	$ (292,263)	$ 1,838,980
Balance (in shares) at Dec. 31, 2017	96,019
Increase (decrease) in shareholders' equity
Net Income			157,402			157,402
Dividends:
Dividends			(49,598)			(49,598)
Purchase of treasury stock					(57)	(57)
Exercise of stock options	$ 64	1,094				1,158
Exercise of stock options (in shares)	64
Stock compensation expense recognized in earnings		755				755
Repurchase of outstanding warrant		(29,005)				(29,005)
Other comprehensive income (loss), net of tax:
Net change in unrealized gains and losses on available for sale securities, net of reclassification adjustments				(60,790)		(60,790)
Balance at Sep. 30, 2018	$ 96,083	144,660	2,005,606	(95,184)	(292,320)	1,858,845
Balance (in shares) at Sep. 30, 2018	96,083
Dividends:
Cumulative adjustment for adoption of new accounting standards			(170)	170
Balance at Jun. 30, 2018	$ 96,066	173,057	1,982,464	(79,537)	(292,320)	1,879,730
Balance (in shares) at Jun. 30, 2018	96,066
Increase (decrease) in shareholders' equity
Net Income			51,095			51,095
Dividends:
Dividends			(27,783)			(27,783)
Exercise of stock options	$ 17	323				340
Exercise of stock options (in shares)	17
Stock compensation expense recognized in earnings		285				285
Repurchase of outstanding warrant		(29,005)				(29,005)
Other comprehensive income (loss), net of tax:
Net change in unrealized gains and losses on available for sale securities, net of reclassification adjustments				(15,817)		(15,817)
Balance at Sep. 30, 2018	$ 96,083	144,660	2,005,606	(95,184)	(292,320)	1,858,845
Balance (in shares) at Sep. 30, 2018	96,083
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			153,370			153,370
Dividends:
Dividends			(68,669)			(68,669)
Purchase of treasury stock					(17,817)	(17,817)
Exercise of stock options	$ 88	1,429				1,517
Exercise of stock options (in shares)	88
Stock compensation expense recognized in earnings		744				744
Other comprehensive income (loss), net of tax:
Net change in unrealized gains and losses on available for sale securities, net of reclassification adjustments				59,197		59,197
Balance at Sep. 30, 2019	$ 96,192	147,456	2,148,835	4,563	(329,122)	2,067,924
Balance (in shares) at Sep. 30, 2019	96,192
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
Increase (decrease) in shareholders' equity
Net Income			51,444			51,444
Dividends:
Dividends			(35,848)			(35,848)
Purchase of treasury stock					(17,689)	(17,689)
Exercise of stock options	$ 41	602				643
Exercise of stock options (in shares)	41
Stock compensation expense recognized in earnings		240				240
Other comprehensive income (loss), net of tax:
Net change in unrealized gains and losses on available for sale securities, net of reclassification adjustments				8,815		8,815
Balance at Sep. 30, 2019	$ 96,192	$ 147,456	$ 2,148,835	$ 4,563	$ (329,122)	$ 2,067,924
Balance (in shares) at Sep. 30, 2019	96,192