Consolidated Condensed Statements of Stockholders' Equity (Unaudited) - USD ($) $ in Thousands	Common Stock [Member]	Additional Paid-in Capital [Member]	Retained Earnings [Member]	Total
Balance (in shares) at Dec. 31, 2017	54,533,000
Balance at Dec. 31, 2017	$ 545	$ 76,413	$ 448,542	$ 525,500
Adoption of accounting standard at Dec. 31, 2017			485	485
Net income			10,331	10,331
Issuance of common stock from share-based payment arrangement exercises and vesting of performance unit awards (in shares)	57,000
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)
Balance (in shares) at Mar. 31, 2018	54,590,000
Balance at Mar. 31, 2018	$ 546	76,915	457,993	535,454
Net income			44,696	44,696
Issuance of common stock from share-based payment arrangement exercises and vesting of performance unit awards (in shares)	77,000
Issuance of common stock from share-based payment arrangement exercises and vesting of performance unit awards	$ 1	651		652
Share-based payment arrangement compensation expense		3,002		3,002
Dividends on common stock			(4,094)	(4,094)
Repurchase and retirement of common stock (in shares)	(200,000)
Repurchase and retirement of common stock	$ (2)	(3,754)		(3,756)
Balance (in shares) at Dec. 31, 2018	54,467,000
Balance at Dec. 31, 2018	$ 545	76,814	498,595	575,954
Net income			13,546	13,546
Issuance of common stock from share-based payment arrangement exercises and vesting of performance unit awards (in shares)	105,000
Issuance of common stock from share-based payment arrangement exercises and vesting of performance unit awards	$ 1	291		292
Employee taxes paid in exchange for shares withheld		(784)		(784)
Share-based payment arrangement compensation expense		366		366
Dividends on common stock			(1,637)	$ (1,637)
Repurchase and retirement of common stock (in shares)				0
Balance (in shares) at Mar. 31, 2019	54,572,000
Balance at Mar. 31, 2019	$ 546	$ 76,687	$ 510,504	$ 587,737