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, 2014	79,814,553
Balance at Dec. 31, 2014	$ 840	$ 789,519	$ (5,569)	$ 943,834	$ (125,736)	$ 1,602,888
Dividend Reinvestment Plan (in shares)	148,582					148,582
Dividend Reinvestment Plan	$ 2	4,173				$ 4,175
Restricted stock units vested (in shares)	18,955
Restricted stock units vested
Warrant exercised (in shares)	369
Warrant exercised
Shares withheld related to net share settlement of RSUs		(227)				(227)
Stock issued to directors (in shares)	17,974
Stock issued to directors		495				$ 495
Stock options exercised (in shares)	214,580					214,580
Stock options exercised	$ 2	5,012				$ 5,014
Equity consideration for acquisition (in shares)	2,580,353
Equity consideration for acquisition	$ 26	82,743				82,769
Purchases of treasury stock (in shares)	(1,989,250)
Purchases of treasury stock					(59,412)	(59,412)
Tax short-fall from stock options		(5,348)				(5,348)
Stock -based compensation		4,455				4,455
Cash dividends				(45,283)		(45,283)
Change in other comprehensive loss			(2,857)			(2,857)
Net income				161,109		161,109
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
Equity consideration for acquisition (in shares)	926,192
Equity consideration for acquisition	$ 9	34,853				34,862
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
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