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, Shares at Dec. 31, 2014		31,009,026
Balance at beginning of period at Dec. 31, 2014		$ 31,008	$ 81,337	$ 492,776	$ 4,193
Shares issued, Shares		242,760
Shares issued, Value		$ 242	3,023
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, Shares		(56,894)
Shares acquired and canceled, Value		$ (56)		(1,626)
Balance at end of period, Shares at Dec. 31, 2015	31,194,892	31,194,892
Balance at end of period at Dec. 31, 2015	$ 655,510	$ 31,194	87,268	535,521	1,527
Shares issued, Shares		626,978
Shares issued, Value		$ 628	9,305
Net income	70,674			70,674
Net change	$ (1,433)				(1,433)
Tax effect of stock options			3,521
Dividends on common stock				(23,124)
Stock-based compensation arrangements			1,636
Shares acquired and canceled, Shares		(200,000)
Shares acquired and canceled, Value		$ (200)		(5,423)
Balance at end of period, Shares at Dec. 31, 2016	31,621,870	31,621,870
Balance at end of period at Dec. 31, 2016	$ 711,094	$ 31,622	101,730	577,648	94
Shares issued, Shares		272,693
Shares issued, Value		$ 273	4,571
Net income	86,439			86,439
Net change	$ (2,421)				(2,421)
Dividends on common stock				(25,507)
Stock-based compensation arrangements			1,180
Balance at end of period, Shares at Dec. 31, 2017	31,894,563	31,894,563
Balance at end of period at Dec. 31, 2017	$ 775,629	$ 31,895	$ 107,481	$ 638,580	$ (2,327)