Consolidated Condensed Statements of Stockholders' Equity (Unaudited) - USD ($) shares in Thousands, $ in Thousands	Common Stock [Member]	Additional Paid-in Capital [Member]	Retained Earnings [Member]	Total
Balance (in shares) at Dec. 31, 2016	54,392
Balance at Dec. 31, 2016	$ 544	$ 74,175	$ 362,619	$ 437,338
Net income			17,355	17,355
Issuance of common stock from share-based payment arrangement exercises and vesting of performance unit awards (in shares)	122
Issuance of common stock from share-based payment arrangement exercises and vesting of performance unit awards	$ 1	882		883
Employee taxes paid in exchange for shares withheld		(47)		(47)
Share-based payment arrangement compensation expense		730		730
Dividends on common stock			(1,635)	(1,635)
Balance (in shares) at Jun. 30, 2017	54,514
Balance at Jun. 30, 2017	$ 545	75,740	378,339	454,624
Net income			72,929	72,929
Issuance of common stock from share-based payment arrangement exercises and vesting of performance unit awards (in shares)	19
Issuance of common stock from share-based payment arrangement exercises and vesting of performance unit awards		207		207
Share-based payment arrangement compensation expense		520		520
Dividends on common stock			(2,726)	(2,726)
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
Net income			24,033	24,033
Issuance of common stock from share-based payment arrangement exercises and vesting of performance unit awards (in shares)	126
Issuance of common stock from share-based payment arrangement exercises and vesting of performance unit awards	$ 2	847		849
Employee taxes paid in exchange for shares withheld		(104)		(104)
Share-based payment arrangement compensation expense		1,210		1,210
Dividends on common stock			(2,731)	(2,731)
Balance (in shares) at Jun. 30, 2018	54,659
Balance at Jun. 30, 2018	$ 547	$ 78,366	470,329	549,242
Adoption of accounting standard (Note 2)			$ 485	$ 485