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			8,214	8,214
Issuance of common stock from share-based payment arrangement exercises and vesting of performance unit awards (in shares)	83
Issuance of common stock from share-based payment arrangement exercises and vesting of performance unit awards	$ 1	614		615
Employee taxes paid in exchange for shares withheld		(47)		(47)
Share-based payment arrangement compensation expense		149		149
Dividends on common stock			(817)	(817)
Balance (in shares) at Mar. 31, 2017	54,475
Balance at Mar. 31, 2017	$ 545	74,891	370,016	445,452
Net income			82,070	82,070
Issuance of common stock from share-based payment arrangement exercises and vesting of performance unit awards (in shares)	58
Issuance of common stock from share-based payment arrangement exercises and vesting of performance unit awards		475		475
Share-based payment arrangement compensation expense		1,101		1,101
Dividends on common stock			(3,544)	(3,544)
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			10,331	10,331
Issuance of common stock from share-based payment arrangement exercises and vesting of performance unit awards (in shares)	57
Issuance of common stock from share-based payment arrangement exercises and vesting of performance unit awards	$ 1	285		286
Employee taxes paid in exchange for shares withheld		(104)		(104)
Share-based payment arrangement compensation expense		321		321
Dividends on common stock			(1,365)	(1,365)
Adoption of accounting standard (Note 2) at Dec. 31, 2017			485	485
Balance (in shares) at Mar. 31, 2018	54,590
Balance at Mar. 31, 2018	$ 546	$ 76,915	$ 457,993	$ 535,454