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, 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 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
Balance (in shares) at Dec. 31, 2017	54,533
Balance at Dec. 31, 2017	$ 545	76,413	448,542	525,500
Net income				39,290
Adoption of accounting standard at Dec. 31, 2017			485	485
Balance (in shares) at Sep. 30, 2018	54,663
Balance at Sep. 30, 2018	$ 547	79,304	484,219	564,070
Balance (in shares) at Mar. 31, 2018	54,590
Balance at Mar. 31, 2018	$ 546	76,915	457,993	535,454
Net income			13,702	13,702
Issuance of common stock from share-based payment arrangement exercises and vesting of performance unit awards (in shares)	69
Issuance of common stock from share-based payment arrangement exercises and vesting of performance unit awards	$ 1	562		563
Share-based payment arrangement compensation expense		889		889
Dividends on common stock			(1,366)	(1,366)
Balance (in shares) at Jun. 30, 2018	54,659
Balance at Jun. 30, 2018	$ 547	78,366	470,329	549,242
Net income			15,257	15,257
Issuance of common stock from share-based payment arrangement exercises and vesting of performance unit awards (in shares)	4
Issuance of common stock from share-based payment arrangement exercises and vesting of performance unit awards		46		46
Share-based payment arrangement compensation expense		892		892
Dividends on common stock			(1,367)	(1,367)
Balance (in shares) at Sep. 30, 2018	54,663
Balance at Sep. 30, 2018	$ 547	79,304	484,219	564,070
Balance (in shares) at Dec. 31, 2018	54,467
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
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)
Balance (in shares) at Mar. 31, 2019	54,572
Balance at Mar. 31, 2019	$ 546	76,687	510,504	587,737
Balance (in shares) at Dec. 31, 2018	54,467
Balance at Dec. 31, 2018	$ 545	76,814	498,595	575,954
Net income				45,323
Balance (in shares) at Sep. 30, 2019	54,687
Balance at Sep. 30, 2019	$ 547	78,840	503,470	582,857
Balance (in shares) at Mar. 31, 2019	54,572
Balance at Mar. 31, 2019	$ 546	76,687	510,504	587,737
Net income			15,190	15,190
Issuance of common stock from share-based payment arrangement exercises and vesting of performance unit awards (in shares)	61
Issuance of common stock from share-based payment arrangement exercises and vesting of performance unit awards		466		466
Share-based payment arrangement compensation expense		636		636
Dividends on common stock			(1,639)	(1,639)
Balance (in shares) at Jun. 30, 2019	54,633
Balance at Jun. 30, 2019	$ 546	77,789	524,055	602,390
Net income			16,587	16,587
Issuance of common stock from share-based payment arrangement exercises and vesting of performance unit awards (in shares)	54
Issuance of common stock from share-based payment arrangement exercises and vesting of performance unit awards	$ 1	658		659
Share-based payment arrangement compensation expense		393		393
Dividends on common stock			(37,172)	(37,172)
Balance (in shares) at Sep. 30, 2019	54,687
Balance at Sep. 30, 2019	$ 547	$ 78,840	$ 503,470	$ 582,857