Consolidated Statements of Changes in Stockholders' Equity - USD ($) $ in Thousands	Common Stock Outstanding [Member]	Additional Paid-in Capital [Member]	AOCI Attributable to Parent [Member]	Retained Earnings [Member]	Treasury Stock [Member]	Total
Balance (in shares) at Dec. 31, 2017	80,893,379
Beginning balance, loss, net of tax at Dec. 31, 2017	$ 891	$ 932,874	$ (2,511)	$ 1,281,639	$ (239,589)	$ 1,973,304
Cumulative effect of changes in accounting principles	0	0	(8,556)	8,556	0	0
Reclassification of tax effects in accumulated other comprehensive income resulting from the new corporate income tax rate	$ 0	0	(515)	515	0	$ 0
Dividend Reinvestment Plan (in shares)	69,084					69,084
Dividend Reinvestment Plan	$ 1	2,821	0	0	0	$ 2,822
Restricted stock units vested (in shares)	125,207
Restricted stock units vested	$ 1	0	0	0	0	1
Warrants exercised (in shares)	476,064
Warrants exercised	$ 5	(5)	0	0	0	0
Shares withheld related to net share settlement of RSUs	$ 0	(2,440)	0	0	0	(2,440)
Stock issued to directors (in shares)	16,060
Stock issued to directors	$ 0	649	0	0	0	$ 649
Stock options exercised (in shares)	35,880					35,880
Stock options exercised	$ 0	838	0	0	0	$ 838
Purchases of treasury stock (in shares)	(1,113,726)
Purchases of treasury stock	$ 0	0	0	0	(42,648)	(42,648)
Stock -based compensation	0	7,325	0	0	0	7,325
Cash dividends	0	0	0	(83,446)	0	(83,446)
Other comprehensive income (loss)	0	0	(6,424)	0	0	(6,424)
Net income	$ 0	0	0	271,885	0	271,885
Balance (in shares) at Dec. 31, 2018	80,501,948
Ending balance, loss, net of tax at Dec. 31, 2018	$ 898	942,062	(18,006)	1,479,149	(282,237)	$ 2,121,866
Dividend Reinvestment Plan (in shares)	93,143					93,143
Dividend Reinvestment Plan	$ 1	3,365	0	0	0	$ 3,366
Restricted stock units vested (in shares)	123,762
Restricted stock units vested	$ 1	0	0	0	0	1
Shares withheld related to net share settlement of RSUs	$ 0	(2,311)	0	0	0	(2,311)
Stock issued to directors (in shares)	21,160
Stock issued to directors	$ 0	749	0	0	0	749
Purchases of treasury stock (in shares)	(1,010,594)
Purchases of treasury stock	$ 0	0	0	0	(36,301)	(36,301)
Stock -based compensation	0	6,601	0	0	0	6,601
Cash dividends	0	0	0	(99,131)	0	(99,131)
Other comprehensive income (loss)	0	0	20,308	0	0	20,308
Net income	$ 0	0	0	279,135	0	279,135
Balance (in shares) at Dec. 31, 2019	79,729,419
Ending balance, loss, net of tax at Dec. 31, 2019	$ 900	950,466	2,302	1,659,153	(318,538)	$ 2,294,283
Dividend Reinvestment Plan (in shares)	358,157					358,157
Dividend Reinvestment Plan	$ 4	9,773	0	0	0	$ 9,777
Restricted stock units vested (in shares)	189,557
Restricted stock units vested	$ 2	0	0	0	0	2
Shares withheld related to net share settlement of RSUs	$ 0	(1,911)	0	0	0	(1,911)
Stock issued to directors (in shares)	31,110
Stock issued to directors	$ 0	800	0	0	0	800
Purchases of treasury stock (in shares)	(799,978)
Purchases of treasury stock	$ 0	0	0	0	(23,593)	(23,593)
Stock -based compensation	0	5,606	0	0	0	5,606
Cash dividends	0	0	0	(98,688)	0	(98,688)
Other comprehensive income (loss)	0	0	3,008	0	0	3,008
Net income	$ 0	0	0	228,860	0	228,860
Balance (in shares) at Dec. 31, 2020	79,508,265
Ending balance, loss, net of tax at Dec. 31, 2020	$ 906	$ 964,734	$ 5,310	$ 1,789,325	$ (342,131)	$ 2,418,144