Consolidated Statements of Changes in Stockholders' Equity - 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, 2015	80,806,116
Balance at Dec. 31, 2015	$ 870	$ 880,822	$ (8,426)	$ 1,059,660	$ (185,148)	$ 1,747,778
Dividend Reinvestment Plan (in shares)	72,231					72,231
Dividend Reinvestment Plan	$ 1	2,276				$ 2,277
Restricted stock units vested (in shares)	10,325
Restricted stock units vested
Warrant exercised (in shares)	388,001
Warrant exercised	$ 4	(4)
Shares withheld related to net share settlement of RSUs		(103)				(103)
Stock issued to directors (in shares)	19,602
Stock issued to directors		550				$ 550
Stock options exercised (in shares)	327,830					327,830
Stock options exercised	$ 3	7,658				$ 7,661
Purchases of treasury stock (in shares)	(2,013,828)
Purchases of treasury stock					(54,441)	(54,441)
Tax short-fall from stock options		(132)				(132)
Stock -based compensation		4,413				4,413
Cash dividends				(59,274)		(59,274)
Change in other comprehensive loss			4,711			4,711
Net income				175,099		175,099
Balance (in shares) at Dec. 31, 2016	79,610,277
Balance at Dec. 31, 2016	$ 878	895,480	(3,715)	1,175,485	(239,589)	$ 1,828,539
Dividend Reinvestment Plan (in shares)	65,044					65,044
Dividend Reinvestment Plan	$ 1	2,527				$ 2,528
Restricted stock units vested (in shares)	224,995
Restricted stock units vested	$ 2					2
Warrant exercised (in shares)	4,681
Warrant exercised
Shares withheld related to net share settlement of RSUs		(6,813)				(6,813)
Stock issued to directors (in shares)	15,400
Stock issued to directors		550				$ 550
Stock options exercised (in shares)	46,790					46,790
Stock options exercised	$ 1	1,093				$ 1,094
Stock -based compensation		5,184				5,184
Cash dividends				(69,888)		(69,888)
Change in other comprehensive loss			1,204			1,204
Net income				176,042		176,042
Equity consideration for acquisition (in shares)	926,192
Equity consideration for acquisition	$ 9	34,853				34,862
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)	69,084					69,084
Dividend Reinvestment Plan	$ 1	2,821				$ 2,822
Restricted stock units vested (in shares)	125,207
Restricted stock units vested	$ 1					1
Warrant exercised (in shares)	476,064
Warrant exercised	$ 5	(5)
Shares withheld related to net share settlement of RSUs		(2,440)				(2,440)
Stock issued to directors (in shares)	16,060
Stock issued to directors		649				$ 649
Stock options exercised (in shares)	35,880					35,880
Stock options exercised		838				$ 838
Purchases of treasury stock (in shares)	(1,113,726)
Purchases of treasury stock					(42,648)	(42,648)
Stock -based compensation		7,325				7,325
Cash dividends				(83,446)		(83,446)
Change in other comprehensive loss			(6,424)			(6,424)
Net income				271,885		271,885
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
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