Condensed 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, 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)	70,798
Dividend Reinvestment Plan	$ 1	2,521	0	0	0	2,522
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 -based compensation	0	4,848	0	0	0	4,848
Cash dividends	0	0	0	(74,422)	0	(74,422)
Total other comprehensive (loss)/income, net of tax	0	0	17,173	0	0	17,173
Net income	$ 0	0	0	211,758	0	211,758
Purchases of treasury stock (in shares)	(1,010,594)
Purchases of treasury stock	$ 0	0	0	0	(36,301)	(36,301)
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
Ending balance, loss, net of tax 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
Beginning balance, loss, net of tax 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
Stock -based compensation	0	1,789	0	0	0	1,789
Cash dividends	0	0	0	(24,701)	0	(24,701)
Total other comprehensive (loss)/income, net of tax	0	0	525	0	0	525
Net income	$ 0	0	0	72,835	0	72,835
Purchases of treasury stock (in shares)	(135,000)
Purchases of treasury stock	$ 0	0	0	0	(4,692)	(4,692)
Balance (in shares) at Sep. 30, 2019	79,706,511
Ending balance, loss, net of tax at Sep. 30, 2019	$ 900	947,880	(833)	1,616,485	(318,538)	2,245,894
Balance (in shares) at Dec. 31, 2019	79,729,419
Beginning 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)	109,988
Dividend Reinvestment Plan	$ 2	2,571	0	0	0	2,573
Restricted stock units vested (in shares)	188,879
Restricted stock units vested	$ 2	0	0	0	0	2
Shares withheld related to net share settlement of RSUs	0	(1,903)	0	0	0	(1,903)
Stock -based compensation	0	3,808	0	0	0	3,808
Cash dividends	0	0	0	(74,014)	0	(74,014)
Total other comprehensive (loss)/income, net of tax	0	0	4,087	0	0	4,087
Net income	$ 0	0	0	157,967	0	157,967
Purchases of treasury stock (in shares)	(400,000)
Purchases of treasury stock	$ 0	0	0	0	(12,880)	(12,880)
Stock issued to directors (in shares)	31,110
Stock issued to directors	$ 0	800	0	0	0	800
Balance (in shares) at Sep. 30, 2020	79,659,396
Ending balance, loss, net of tax at Sep. 30, 2020	$ 904	955,742	6,389	1,743,106	(331,418)	2,374,723
Balance (in shares) at Jun. 30, 2020	79,619,984
Beginning balance, loss, net of tax at Jun. 30, 2020	$ 903	953,616	8,353	1,710,994	(331,418)	2,342,448
Dividend Reinvestment Plan (in shares)	37,420
Dividend Reinvestment Plan	$ 1	866	0	0	0	867
Restricted stock units vested (in shares)	1,992
Restricted stock units vested	$ 0	0	0	0	0	0
Shares withheld related to net share settlement of RSUs	0	(4)	0	0	0	(4)
Stock -based compensation	0	1,264	0	0	0	1,264
Cash dividends	0	0	0	(24,682)	0	(24,682)
Total other comprehensive (loss)/income, net of tax	0	0	(1,964)	0	0	(1,964)
Net income	$ 0	0	0	56,794	0	56,794
Balance (in shares) at Sep. 30, 2020	79,659,396
Ending balance, loss, net of tax at Sep. 30, 2020	$ 904	$ 955,742	$ 6,389	$ 1,743,106	$ (331,418)	$ 2,374,723