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
Beginning balance, loss, net of tax at Dec. 31, 2017	$ 891	$ 932,874	$ (2,511)	$ 1,281,639	$ (239,589)	$ 1,973,304
Dividend Reinvestment Plan (in shares)	30,557
Dividend Reinvestment Plan	$ 0	1,325	0	0	0	1,325
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
Stock-based compensation	0	3,404	0	0	0	3,404
Cash dividends	0	0	0	(38,966)	0	(38,966)
Change in other comprehensive loss	0	0	(11,336)	0	0	(11,336)
Net income	0	0	0	137,480	0	137,480
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
Warrants exercised (in shares)	190,336
Warrants exercised	$ 2	(2)	0	0	0	$ 0
Stock options exercised (in shares)	35,880					35,880
Stock options exercised	$ 0	838	0	0	0	$ 838
Balance (in shares) at Jun. 30, 2018	81,255,683
Ending balance, loss, net of tax at Jun. 30, 2018	$ 895	937,224	(22,921)	1,389,224	(239,589)	2,064,833
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
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
Balance (in shares) at Mar. 31, 2018	81,206,998
Beginning balance, loss, net of tax at Mar. 31, 2018	$ 894	934,335	(20,906)	1,335,058	(239,589)	2,009,792
Dividend Reinvestment Plan (in shares)	15,286
Dividend Reinvestment Plan	$ 0	661	0	0	0	661
Restricted stock units vested (in shares)	17,339
Restricted stock units vested	$ 1	0	0	0	0	1
Shares withheld related to net share settlement of RSUs	$ 0	(327)	0	0	0	(327)
Stock issued to directors (in shares)	16,060
Stock issued to directors	$ 0	650	0	0	0	650
Stock-based compensation	0	1,905	0	0	0	1,905
Cash dividends	0	0	0	(19,498)	0	(19,498)
Change in other comprehensive loss	0	0	(2,015)	0	0	(2,015)
Net income	$ 0	0	0	73,664	0	73,664
Balance (in shares) at Jun. 30, 2018	81,255,683
Ending balance, loss, net of tax at Jun. 30, 2018	$ 895	937,224	(22,921)	1,389,224	(239,589)	2,064,833
Balance (in shares) at Dec. 31, 2018	80,501,948
Beginning 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)	47,290
Dividend Reinvestment Plan	$ 1	1,680	0	0	0	1,681
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
Purchases of treasury stock (in shares)	(875,594)
Purchases of treasury stock	$ 0	0	0	0	(31,609)	(31,609)
Stock-based compensation	0	3,059	0	0	0	3,059
Cash dividends	0	0	0	(49,721)	0	(49,721)
Change in other comprehensive loss	0	0	16,648	0	0	16,648
Net income	$ 0	0	0	138,923	0	138,923
Balance (in shares) at Jun. 30, 2019	79,818,003
Ending balance, loss, net of tax at Jun. 30, 2019	$ 900	945,250	(1,358)	1,568,351	(313,846)	2,199,297
Balance (in shares) at Mar. 31, 2019	80,362,840
Beginning balance, loss, net of tax at Mar. 31, 2019	$ 899	942,838	(10,431)	1,520,861	(290,838)	2,163,329
Dividend Reinvestment Plan (in shares)	24,345
Dividend Reinvestment Plan	$ 1	845	0	0	0	846
Restricted stock units vested (in shares)	51,552
Restricted stock units vested	$ 0	0	0	0	0	0
Shares withheld related to net share settlement of RSUs	$ 0	(707)	0	0	0	(707)
Stock issued to directors (in shares)	21,160
Stock issued to directors	$ 0	749	0	0	0	749
Purchases of treasury stock (in shares)	(641,894)
Purchases of treasury stock	$ 0	0	0	0	(23,008)	(23,008)
Stock-based compensation	0	1,525	0	0	0	1,525
Cash dividends	0	0	0	(24,754)	0	(24,754)
Change in other comprehensive loss	0	0	9,073	0	0	9,073
Net income	$ 0	0	0	72,244	0	72,244
Balance (in shares) at Jun. 30, 2019	79,818,003
Ending balance, loss, net of tax at Jun. 30, 2019	$ 900	$ 945,250	$ (1,358)	$ 1,568,351	$ (313,846)	$ 2,199,297