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, 2013		$ 15,334	$ 88,803	$ 448,953	$ 3,907
Shares issued		115	3,174
Net income	$ 48,138			48,138
Net change	659				659
Tax effect of stock options			665
Dividends on common stock				(14,789)
Stock-based compensation arrangements			1,224
Balance at end of period at Sep. 30, 2014	596,183	15,449	93,866	482,302	4,566
Balance at beginning of period at Jun. 30, 2014		15,399	91,447	468,761	4,898
Shares issued		50	1,554
Net income	18,793			18,793
Net change	(332)				(332)
Tax effect of stock options			417
Dividends on common stock				(5,252)
Stock-based compensation arrangements			448
Balance at end of period at Sep. 30, 2014	596,183	15,449	93,866	482,302	4,566
Balance at beginning of period at Dec. 31, 2014	609,314	15,504	96,841	492,776	4,193
Shares issued		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		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