Consolidated Statements of Stockholders' Equity (Unaudited) - USD ($) $ in Thousands	Total	COMMON STOCK [Member]	CAPITAL SURPLUS [Member]	RETAINED EARNINGS [Member]	RETAINED EARNINGS [Member] CUMULATIVE EFFECT, PERIOD OF ADOPTION, ADJUSTMENT [Member]	ACCUMULATED OTHER COMPREHENSIVE INCOME (LOSS) [Member]
Balance at beginning of period at Dec. 31, 2018		$ 32,604	$ 149,709	$ 722,615		$ (2,139)
Shares issued for stock options		40	873
Net income	$ 99,372			$ 99,372
Accounting Standards Update [Extensible List]				us-gaap:AccountingStandardsUpdate201613Member
Dividends on common stock				$ (29,978)
Stock-based compensation arrangements			888
Net change						5,768
Balance at end of period at Sep. 30, 2019	979,752	32,644	151,470	792,009		3,629
Balance at beginning of period at Jun. 30, 2019		32,640	150,995	769,090		3,655
Shares issued for stock options		4	128
Net income	33,368			33,368
Dividends on common stock				(10,449)
Stock-based compensation arrangements			347
Net change						(26)
Balance at end of period at Sep. 30, 2019	979,752	32,644	151,470	792,009		3,629
Balance at beginning of period at Dec. 31, 2019	1,004,989	32,694	153,353	815,488	$ 2,270	3,454
Shares issued for stock options		44	813
Shares acquired and canceled		(59)		(3,039)
Net income	64,228			64,228
Dividends on common stock				(32,013)
Stock-based compensation arrangements			1,265
Net change						5,254
Balance at end of period at Sep. 30, 2020	1,043,752	32,679	155,431	846,934		8,708
Balance at beginning of period at Jun. 30, 2020		32,663	154,692	837,154		9,690
Shares issued for stock options		16	306
Net income	20,890			20,890
Dividends on common stock				(11,110)
Stock-based compensation arrangements			433
Net change						(982)
Balance at end of period at Sep. 30, 2020	$ 1,043,752	$ 32,679	$ 155,431	$ 846,934		$ 8,708