Consolidated Statements of Changes in Stockholders' Equity (Unaudited) - 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, 2017	56,123,228
Beginning balance 1 at Dec. 31, 2017	$ 561	$ 1,144,570	$ 258,164	$ 3,992	$ 1,407,287
Cumulative effect of newly adopted accounting standards (Note 2) at Dec. 31, 2017			5,766	(3,992)	1,774
Adjusted balance at Dec. 31, 2017	561	1,144,570	263,930		1,409,061
Net Income			38,764		38,764
Shares issued upon exercise of stock options and awards of restricted stock (in shares)	97,783
Shares issued upon exercise of stock options and awards of restricted stock	$ 1	281			282
Cash dividends declared			(16,865)		(16,865)
Stock-based compensation expense		1,251			1,251
Forfeiture of restricted stock (in shares)	(1,368)
Forfeiture of restricted stock
Balance (in shares) at Mar. 31, 2018	56,219,643
Ending balance at Mar. 31, 2018	$ 562	1,146,102	285,829		1,432,493
Balance (in shares) at Dec. 31, 2018	56,615,352
Beginning balance 1 at Dec. 31, 2018	$ 566	1,168,442	406,992		1,576,000
Cumulative effect of newly adopted accounting standards (Note 2) at Dec. 31, 2018			(67)		(67)
Adjusted balance at Dec. 31, 2018	566	1,168,442	406,925		1,575,933
Net Income			40,550		40,550
Shares issued upon exercise of stock options and awards of restricted stock (in shares)	372,344
Shares issued upon exercise of stock options and awards of restricted stock	$ 4	7,083			7,087
Cash dividends declared			(17,019)		(17,019)
Stock dividend declared (in shares)	4,534,908
Stock dividend declared	$ 45	138,950	(139,091)		(96)
Stock-based compensation expense		4,251			4,251
Forfeiture of restricted stock (in shares)	(1,714)
Forfeiture of restricted stock
Balance (in shares) at Mar. 31, 2019	61,520,890
Ending balance at Mar. 31, 2019	$ 615	$ 1,318,726	$ 291,365		$ 1,610,706