Consolidated Statements of Stockholders' Equity - USD ($) $ in Thousands	Total	CUMULATIVE EFFECT, PERIOD OF ADOPTION, ADJUSTMENT [Member]	Common Stock [Member]	CAPITAL SURPLUS [Member]	RETAINED EARNINGS [Member]	RETAINED EARNINGS [Member] CUMULATIVE EFFECT, PERIOD OF ADOPTION, ADJUSTMENT [Member]	ACCUMULATED OTHER COMPREHENSIVE INCOME [Member]	ACCUMULATED OTHER COMPREHENSIVE INCOME [Member] CUMULATIVE EFFECT, PERIOD OF ADOPTION, ADJUSTMENT [Member]
Balance at beginning of period, Shares at Dec. 31, 2017			31,894,563
Balance at beginning of period at Dec. 31, 2017			$ 31,895	$ 107,481	$ 638,580	$ (618)	$ (2,327)	$ 618
Accounting Standards Update Extensible List					us-gaap:AccountingStandardsUpdate201613Member		us-gaap:AccountingStandardsUpdate201613Member
Shares issued for stock options, Shares			127,816
Shares issued for stock options, Value			$ 127	2,111
Shares issued for acquisitions, Shares			732,811
Shares issued for acquisitions, Value			$ 733	38,765
Net income	$ 125,814				$ 125,814
Net change	$ (430)						$ (430)
Dividends on common stock					(33,398)
Stock-based compensation arrangements				1,352
Shares acquired and canceled, Shares			(151,264)
Shares acquired and canceled, Value			$ (151)		(7,763)
Balance at end of period, Shares at Dec. 31, 2018	32,603,926		32,603,926
Balance at end of period at Dec. 31, 2018	$ 902,789		$ 32,604	149,709	722,615		(2,139)
Shares issued for stock options, Shares			117,012
Shares issued for stock options, Value			$ 117	2,367
Net income	134,879				134,879
Net change	$ 5,593						5,593
Dividends on common stock					(40,432)
Stock-based compensation arrangements				1,277
Shares acquired and canceled, Shares			(26,670)
Shares acquired and canceled, Value			$ (27)		(1,574)
Balance at end of period, Shares at Dec. 31, 2019	32,694,268		32,694,268
Balance at end of period at Dec. 31, 2019	$ 1,004,989		$ 32,694	153,353	$ 815,488	$ 2,270	3,454
Accounting Standards Update Extensible List		us-gaap:AccountingStandardsUpdate201613Member			us-gaap:AccountingStandardsUpdate201613Member
Shares issued for stock options, Shares			84,868
Shares issued for stock options, Value			$ 85	1,705
Net income	99,586				$ 99,586
Net change	$ 3,976						3,976
Dividends on common stock					(43,144)
Stock-based compensation arrangements				1,516
Shares acquired and canceled, Shares			(59,284)
Shares acquired and canceled, Value			$ (59)		(3,039)
Balance at end of period, Shares at Dec. 31, 2020	32,719,852		32,719,852
Balance at end of period at Dec. 31, 2020	$ 1,067,885		$ 32,720	$ 156,574	$ 871,161		$ 7,430