Consolidated Statements of Stockholders' Equity - USD ($) $ in Thousands	Common Stock [Member]	Additional Paid-in Capital [Member]	Retained Earnings [Member]	AOCI Attributable to Parent [Member]	Total
Balance (in shares) at Dec. 31, 2015	48,888,424
Balance at Dec. 31, 2015	$ 489	$ 915,746	$ 324,342	$ 15,715	$ 1,256,292
Net Income			103,211		103,211
Other comprehensive income, net of tax				6,356	6,356
Total comprehensive income					109,567
Shares issued upon exercise of stock options and awards of restricted stock (in shares)	147,455
Shares issued upon exercise of stock options and awards of restricted stock	$ 1	(1)
Cash dividends declared			(49,021)		(49,021)
Stock dividend declared (in shares)	2,449,211
Stock dividend declared	$ 25	64,488	(64,580)		(67)
Stock-based compensation expense		7,477			7,477
Net income tax deficiency from share-based compensation		(4,178)			(4,178)
Balance (in shares) at Dec. 31, 2016	51,485,090
Balance at Dec. 31, 2016	$ 515	983,532	313,952	22,071	1,320,070
Net Income			141,835		141,835
Other comprehensive income, net of tax				(18,079)	(18,079)
Total comprehensive income					123,756
Shares issued upon exercise of stock options and awards of restricted stock (in shares)	492,408
Shares issued upon exercise of stock options and awards of restricted stock	$ 5	9,496			9,501
Cash dividends declared			(51,779)		(51,779)
Stock dividend declared (in shares)	4,151,722
Stock dividend declared	$ 41	145,685	(145,844)		(118)
Stock-based compensation expense		5,857			5,857
Forfeiture of restricted stock (in shares)	(5,992)
Forfeiture of restricted stock
Balance (in shares) at Dec. 31, 2017	56,123,228
Balance at Dec. 31, 2017	$ 561	1,144,570	258,164	3,992	1,407,287
Cumulative effect of newly adopted accounting standards (Note 2)			5,766	(3,992)	1,774
Adjusted balance	561	1,144,570	263,930		1,409,061
Net Income			210,780		210,780
Other comprehensive income, net of tax
Total comprehensive income					210,780
Shares issued upon exercise of stock options and awards of restricted stock (in shares)	497,024
Shares issued upon exercise of stock options and awards of restricted stock	$ 5	9,855			9,860
Cash dividends declared			(67,718)		(67,718)
Stock-based compensation expense		14,017			14,017
Forfeiture of restricted stock (in shares)	(4,900)
Forfeiture of restricted stock
Balance (in shares) at Dec. 31, 2018	56,615,352
Balance at Dec. 31, 2018	$ 566	$ 1,168,442	$ 406,992		$ 1,576,000