Consolidated Statements of Stockholders' Equity - USD ($) shares in Thousands, $ in Thousands	Common Stock [Member]	Additional Paid-in Capital [Member]	Retained Earnings [Member]	Total
Balance (in shares) at Dec. 31, 2014	55,698
Balance at Dec. 31, 2014	$ 557	$ 87,370	$ 299,999	$ 387,926
Net income			35,745	35,745
Repurchase and retirement of common stock (in shares)	(1,568)
Repurchase and retirement of common stock	$ (16)	(16,166)	7	(16,175)
Issuance of common stock from share-based payment arrangement exercises and vesting of performance unit awards (in shares)	470
Issuance of common stock from share-based payment arrangement exercises and vesting of performance unit awards	$ 5	3,487	(2)	3,490
Tax benefits from share-based payment arrangement exercises		373		373
Share-based payment arrangement compensation expense		1,404		1,404
Dividends on common stock			(3,342)	(3,342)
Balance (in shares) at Dec. 31, 2015	54,600
Balance at Dec. 31, 2015	$ 546	76,468	332,407	409,421
Net income			33,464	33,464
Repurchase and retirement of common stock (in shares)	(759)
Repurchase and retirement of common stock	$ (8)	(7,508)	3	(7,513)
Issuance of common stock from share-based payment arrangement exercises and vesting of performance unit awards (in shares)	551
Issuance of common stock from share-based payment arrangement exercises and vesting of performance unit awards	$ 6	4,413	(3)	4,416
Tax benefits from share-based payment arrangement exercises		46		46
Share-based payment arrangement compensation expense		883		883
Dividends on common stock			(3,252)	(3,252)
Employee taxes paid in exchange for shares withheld		(127)		(127)
Balance (in shares) at Dec. 31, 2016	54,392
Balance at Dec. 31, 2016	$ 544	74,175	362,619	437,338
Net income			90,284	90,284
Issuance of common stock from share-based payment arrangement exercises and vesting of performance unit awards (in shares)	141
Issuance of common stock from share-based payment arrangement exercises and vesting of performance unit awards	$ 1	1,089		1,090
Share-based payment arrangement compensation expense		1,250		1,250
Dividends on common stock			(4,361)	(4,361)
Employee taxes paid in exchange for shares withheld		(47)		(47)
Cash in lieu of fractional shares from stock split		(54)		(54)
Balance (in shares) at Dec. 31, 2017	54,533
Balance at Dec. 31, 2017	$ 545	$ 76,413	$ 448,542	$ 525,500