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 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	0	0	5,766	(3,992)	1,774
Adjusted balance at Dec. 31, 2017	561	1,144,570	263,930	0	1,409,061
Net Income	$ 0	0	38,764	0	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	0	0	282
Cash dividends declared	0	0	(16,865)	0	(16,865)
Stock-based compensation expense	$ 0	1,251	0	0	1,251
Forfeiture of restricted stock (in shares)	(1,368)
Forfeiture of restricted stock	$ 0	0	0	0	0
Balance (in shares) at Mar. 31, 2018	56,219,643
Ending balance at Mar. 31, 2018	$ 562	1,146,102	285,829	0	1,432,493
Balance (in shares) at Dec. 31, 2017	56,123,228
Beginning 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) at Dec. 31, 2017	0	0	5,766	(3,992)	1,774
Adjusted balance at Dec. 31, 2017	$ 561	1,144,570	263,930	0	1,409,061
Net Income					102,663
Balance (in shares) at Jun. 30, 2018	56,435,989
Ending balance at Jun. 30, 2018	$ 564	1,156,477	332,800	0	1,489,841
Balance (in shares) at Mar. 31, 2018	56,219,643
Beginning balance at Mar. 31, 2018	$ 562	1,146,102	285,829	0	1,432,493
Net Income	$ 0	0	63,899	0	63,899
Shares issued upon exercise of stock options and awards of restricted stock (in shares)	216,850
Shares issued upon exercise of stock options and awards of restricted stock	$ 2	5,551	0	0	5,553
Cash dividends declared	0	0	(16,928)	0	(16,928)
Stock-based compensation expense	$ 0	4,824	0	0	4,824
Forfeiture of restricted stock (in shares)	(504)
Forfeiture of restricted stock	$ 0	0	0	0	0
Balance (in shares) at Jun. 30, 2018	56,435,989
Ending balance at Jun. 30, 2018	$ 564	1,156,477	332,800	0	1,489,841
Balance (in shares) at Dec. 31, 2018	56,615,352
Beginning balance at Dec. 31, 2018	$ 566	1,168,442	406,992	0	1,576,000
Cumulative effect of newly adopted accounting standards (Note 2) at Dec. 31, 2018	0	0	(67)	0	(67)
Adjusted balance at Dec. 31, 2018	566	1,168,442	406,925	0	1,575,933
Net Income	$ 0	0	40,550	0	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	0	0	7,087
Cash dividends declared	$ 0	0	(17,019)	0	(17,019)
Stock dividend declared (in shares)	4,534,908
Stock dividend declared	$ 45	138,950	(139,091)		(96)
Stock-based compensation expense	$ 0	4,251	0	0	4,251
Forfeiture of restricted stock (in shares)	(1,714)
Forfeiture of restricted stock	$ 0	0	0	0	0
Balance (in shares) at Mar. 31, 2019	61,520,890
Ending balance at Mar. 31, 2019	$ 615	1,318,726	291,365	0	1,610,706
Balance (in shares) at Dec. 31, 2018	56,615,352
Beginning balance at Dec. 31, 2018	$ 566	1,168,442	406,992	0	1,576,000
Cumulative effect of newly adopted accounting standards (Note 2) at Dec. 31, 2018	0	0	(67)	0	(67)
Adjusted balance at Dec. 31, 2018	$ 566	1,168,442	406,925	0	1,575,933
Net Income					95,143
Balance (in shares) at Jun. 30, 2019	61,922,406
Ending balance at Jun. 30, 2019	$ 619	1,333,095	327,437	0	1,661,151
Balance (in shares) at Mar. 31, 2019	61,520,890
Beginning balance at Mar. 31, 2019	$ 615	1,318,726	291,365	0	1,610,706
Net Income	$ 0	0	54,593	0	54,593
Shares issued upon exercise of stock options and awards of restricted stock (in shares)	405,094
Shares issued upon exercise of stock options and awards of restricted stock	$ 4	10,237	0	0	10,241
Cash dividends declared	0	0	(18,521)	0	(18,521)
Stock-based compensation expense	$ 0	4,132	0	0	4,132
Forfeiture of restricted stock (in shares)	(3,578)
Forfeiture of restricted stock	$ 0	0	0	0	0
Balance (in shares) at Jun. 30, 2019	61,922,406
Ending balance at Jun. 30, 2019	$ 619	$ 1,333,095	$ 327,437	$ 0	$ 1,661,151