Condensed Consolidated Statements of Changes in Stockholders' Equity (Unaudited) - USD ($) $ in Thousands	Common Stock [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)	15,271
Dividend Reinvestment Plan		664				664
Restricted stock units vested (in shares)	72,132
Restricted stock units vested	$ 1					1
Shares withheld related to net share settlement of RSUs		(1,538)				(1,538)
Stock -based compensation		1,499				1,499
Cash dividends				(19,468)		(19,468)
Total other comprehensive (loss)/income, net of tax			(9,321)			(9,324)
Net income				63,816		63,816
Cumulative effect of changes in accounting principles			(8,556)	8,556
Reclassification of tax effects in accumulated other comprehensive income resulting from the new corporate income tax rate			(515)	515
Warrants exercised (in shares)	190,336
Warrants exercised	$ 2	(2)
Stock options exercised (in shares)	35,880					35,880
Stock options exercised		838				$ 838
Balance (in shares) at Mar. 31, 2018	81,206,998
Ending balance, loss, net of tax at Mar. 31, 2018	$ 894	934,335	(20,906)	1,335,058	(239,589)	2,009,792
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
Purchases of treasury stock (in shares)						(913,400)
Purchases of treasury stock						$ (35,100)
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)	22,945
Dividend Reinvestment Plan		835				835
Restricted stock units vested (in shares)	71,647
Restricted stock units vested	$ 1					1
Shares withheld related to net share settlement of RSUs		(1,593)				$ (1,593)
Purchases of treasury stock (in shares)	(233,700)					(233,700)
Purchases of treasury stock					(8,601)	$ (8,601)
Stock -based compensation		1,534				1,534
Cash dividends				(24,967)		(24,967)
Total other comprehensive (loss)/income, net of tax			7,575			7,575
Net income				66,679		66,679
Balance (in shares) at Mar. 31, 2019	80,362,840
Ending balance, loss, net of tax at Mar. 31, 2019	$ 899	$ 942,838	$ (10,431)	$ 1,520,861	$ (290,838)	$ 2,163,329