Consolidated Statements of Stockholders' Equity (Unaudited) - USD ($) $ in Thousands	Total	COMMON STOCK [Member]	CAPITAL SURPLUS [Member]	RETAINED EARNINGS [Member]	ACCUMULATED OTHER COMPREHENSIVE INCOME [Member]
Balance at beginning of period at Dec. 31, 2014		$ 15,504	$ 96,841	$ 492,776	$ 4,193
Shares issued, Value		87	2,065
Net income	$ 50,442			50,442
Net change	(1,596)				(1,596)
Tax effect of stock options			686
Dividends on common stock				(16,180)
Stock-based compensation arrangements			1,243
Balance at end of period at Sep. 30, 2015	646,061	15,591	100,835	527,038	2,597
Balance at beginning of period at Jun. 30, 2015		15,562	99,202	517,028	2,452
Shares issued, Value		29	749
Net income	15,630			15,630
Net change	145				145
Tax effect of stock options			395
Dividends on common stock				(5,620)
Stock-based compensation arrangements			489
Balance at end of period at Sep. 30, 2015	646,061	15,591	100,835	527,038	2,597
Balance at beginning of period at Dec. 31, 2015	655,510	15,597	102,865	535,521	1,527
Shares issued, Value		198	5,634
Net income	52,054			52,054
Net change	458				458
Tax effect of stock options			1,247
Dividends on common stock				(17,113)
Stock-based compensation arrangements			1,266
Shares acquired and canceled, Value		(100)		(5,423)
Balance at end of period at Sep. 30, 2016	693,731	15,695	111,012	565,039	1,985
Balance at beginning of period at Jun. 30, 2016		15,560	105,676	552,991	2,603
Shares issued, Value		135	3,767
Net income	17,982			17,982
Net change	(618)				(618)
Tax effect of stock options			1,204
Dividends on common stock				(5,934)
Stock-based compensation arrangements			365
Balance at end of period at Sep. 30, 2016	$ 693,731	$ 15,695	$ 111,012	$ 565,039	$ 1,985