Consolidated Statements of Stockholders' Equity - 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, 2012		$ 15,242	$ 82,401	$ 415,607	$ 6,317
Balance at beginning of period, Shares at Dec. 31, 2012		15,242,308
Shares issued, Value		$ 158	3,590
Shares issued, Shares		157,155
Net income	$ 54,317			54,317
Net change	(2,410)				(2,410)
Tax effect of stock options			1,452
Dividends on common stock				(18,327)
Stock-based compensation arrangements			1,360
Shares acquired and canceled, Value		$ (66)		(2,644)
Shares acquired and canceled, Shares		(65,841)
Balance at end of period at Dec. 31, 2013	$ 556,997	$ 15,334	88,803	448,953	3,907
Balance at end of period, Shares at Dec. 31, 2013	15,333,622	15,333,622
Shares issued, Value		$ 170	4,742
Shares issued, Shares		170,891
Net income	$ 63,887			63,887
Net change	286				286
Tax effect of stock options			1,620
Dividends on common stock				(20,064)
Stock-based compensation arrangements			1,676
Balance at end of period at Dec. 31, 2014	$ 609,314	$ 15,504	96,841	492,776	4,193
Balance at end of period, Shares at Dec. 31, 2014	15,504,513	15,504,513
Shares issued, Value		$ 121	3,116
Shares issued, Shares		121,380
Net income	$ 66,170			66,170
Net change	(2,666)				(2,666)
Tax effect of stock options			1,191
Dividends on common stock				(21,799)
Stock-based compensation arrangements			1,717
Shares acquired and canceled, Value		$ (28)		(1,626)
Shares acquired and canceled, Shares		(28,447)
Balance at end of period at Dec. 31, 2015	$ 655,510	$ 15,597	$ 102,865	$ 535,521	$ 1,527
Balance at end of period, Shares at Dec. 31, 2015	15,597,446	15,597,446