Condensed Consolidated Statements of Changes in Stockholders' Equity (Unaudited) - 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
Balance at Dec. 31, 2017	$ 891	$ 932,874	$ (2,511)	$ 1,281,639	$ (239,589)	$ 1,973,304
Dividend Reinvestment Plan (in shares)	46,070
Dividend Reinvestment Plan	$ 0	1,982	0	0	0	1,982
Stock-based compensation	0	5,325	0	0	0	5,325
Cash dividends	0	0	0	(58,498)	0	(58,498)
Change in other comprehensive loss	0	0	(12,198)	0	0	(12,198)
Net income	$ 0	0	0	207,237	0	207,237
Warrants exercised (in shares)	315,187
Warrants exercised	$ 3	(3)	0	0	0	0
Restricted stock units vested (in shares)	89,471
Restricted stock units vested	$ 2	0	0	0	0	2
Shares withheld related to net share settlement of RSUs	$ 0	(1,865)	0	0	0	(1,865)
Stock issued to directors (in shares)	16,060
Stock issued to directors	$ 0	650	0	0	0	650
Cumulative effect of changes in accounting principles	0	0	(8,559)	8,559	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
Stock options exercised (in shares)	35,880					35,880
Stock options exercised	$ 0	838	0	0	0	$ 838
Balance (in shares) at Sep. 30, 2018	81,396,047
Balance at Sep. 30, 2018	$ 896	939,801	(23,783)	1,439,452	(239,589)	2,116,777
Balance (in shares) at Jun. 30, 2018	81,255,683
Balance at Jun. 30, 2018	$ 895	937,224	(22,921)	1,389,227	(239,589)	2,064,836
Dividend Reinvestment Plan (in shares)	15,513
Dividend Reinvestment Plan	$ 0	657	0	0	0	657
Stock-based compensation	0	1,921	0	0	0	1,921
Cash dividends	0	0	0	(19,532)	0	(19,532)
Change in other comprehensive loss	0	0	(862)	0	0	(862)
Net income	$ 0	0	0	69,757	0	69,757
Warrants exercised (in shares)	124,851
Warrants exercised	$ 1	(1)	0	0	0	0
Balance (in shares) at Sep. 30, 2018	81,396,047
Balance at Sep. 30, 2018	$ 896	939,801	(23,783)	1,439,452	(239,589)	2,116,777
Balance (in shares) at Dec. 31, 2018	80,501,948
Balance at Dec. 31, 2018	$ 898	942,062	(18,006)	1,479,149	(282,237)	2,121,866
Dividend Reinvestment Plan (in shares)	70,798
Dividend Reinvestment Plan	$ 1	2,521	0	0	0	2,522
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	4,848	0	0	0	4,848
Cash dividends	0	0	0	(74,422)	0	(74,422)
Change in other comprehensive loss	0	0	17,173	0	0	17,173
Net income	$ 0	0	0	211,758	0	211,758
Restricted stock units vested (in shares)	123,199
Restricted stock units vested	$ 1	0	0	0	0	1
Shares withheld related to net share settlement of RSUs	$ 0	(2,300)	0	0	0	(2,300)
Stock issued to directors (in shares)	21,160
Stock issued to directors	$ 0	749	0	0	0	749
Balance (in shares) at Sep. 30, 2019	79,706,511
Balance at Sep. 30, 2019	$ 900	947,880	(833)	1,616,485	(318,538)	2,245,894
Balance (in shares) at Jun. 30, 2019	79,818,003
Balance at Jun. 30, 2019	$ 900	945,250	(1,358)	1,568,351	(313,846)	2,199,297
Dividend Reinvestment Plan (in shares)	23,508
Dividend Reinvestment Plan	$ 0	841	0	0	0	841
Purchases of treasury stock (in shares)	(135,000)
Purchases of treasury stock	$ 0	0	0	0	(4,692)	(4,692)
Stock-based compensation	0	1,789	0	0	0	1,789
Cash dividends	0	0	0	(24,701)	0	(24,701)
Change in other comprehensive loss	0	0	525	0	0	525
Net income	$ 0	0	0	72,835	0	72,835
Balance (in shares) at Sep. 30, 2019	79,706,511
Balance at Sep. 30, 2019	$ 900	$ 947,880	$ (833)	$ 1,616,485	$ (318,538)	$ 2,245,894