Consolidated Condensed Statements of Stockholders' Equity (Unaudited) - USD ($) shares 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	$ 545,000	$ 76,413,000	$ 448,542,000	$ 525,500,000
Net income	$ 0	0	10,331,000	10,331,000
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	$ 1,000	285,000	0	286,000
Employee taxes paid in exchange for shares withheld	0	(104,000)	0	(104,000)
Share-based payment arrangement compensation expense	0	321,000	0	321,000
Dividends on common stock	0	0	(1,365,000)	(1,365,000)
Adoption of accounting standard at Dec. 31, 2017	$ 0	0	485,000	485,000
Balance (in shares) at Mar. 31, 2018	54,590
Balance at Mar. 31, 2018	$ 546,000	76,915,000	457,993,000	535,454,000
Balance (in shares) at Dec. 31, 2017	54,533
Balance at Dec. 31, 2017	$ 545,000	76,413,000	448,542,000	525,500,000
Net income				24,033,000
Adoption of accounting standard at Dec. 31, 2017	$ 0	0	485,000	485,000
Balance (in shares) at Jun. 30, 2018	54,659
Balance at Jun. 30, 2018	$ 547,000	78,366,000	470,329,000	549,242,000
Balance (in shares) at Mar. 31, 2018	54,590
Balance at Mar. 31, 2018	$ 546,000	76,915,000	457,993,000	535,454,000
Net income	$ 0	0	13,702,000	13,702,000
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	$ 1,000	562,000	0	563,000
Share-based payment arrangement compensation expense	0	889,000	0	889,000
Dividends on common stock	$ 0	0	(1,366,000)	(1,366,000)
Balance (in shares) at Jun. 30, 2018	54,659
Balance at Jun. 30, 2018	$ 547,000	78,366,000	470,329,000	549,242,000
Balance (in shares) at Dec. 31, 2018	54,467
Balance at Dec. 31, 2018	$ 545,000	76,814,000	498,595,000	575,954,000
Net income	$ 0	0	13,546,000	13,546,000
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	$ 1,000	291,000	0	292,000
Employee taxes paid in exchange for shares withheld	0	(784,000)	0	(784,000)
Share-based payment arrangement compensation expense	0	366,000	0	366,000
Dividends on common stock	$ 0	0	(1,637,000)	(1,637,000)
Balance (in shares) at Mar. 31, 2019	54,572
Balance at Mar. 31, 2019	$ 546,000	76,687,000	510,504,000	587,737,000
Balance (in shares) at Dec. 31, 2018	54,467
Balance at Dec. 31, 2018	$ 545,000	76,814,000	498,595,000	575,954,000
Net income				28,736,000
Balance (in shares) at Jun. 30, 2019	54,633
Balance at Jun. 30, 2019	$ 546,000	77,789,000	524,055,000	602,390,000
Balance (in shares) at Mar. 31, 2019	54,572
Balance at Mar. 31, 2019	$ 546,000	76,687,000	510,504,000	587,737,000
Net income	$ 0	0	15,190,000	15,190,000
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	$ 0	466,000	0	466,000
Share-based payment arrangement compensation expense	0	636,000	0	636,000
Dividends on common stock	$ 0	0	(1,639,000)	(1,639,000)
Balance (in shares) at Jun. 30, 2019	54,633
Balance at Jun. 30, 2019	$ 546,000	$ 77,789,000	$ 524,055,000	$ 602,390,000