CONDENSED CONSOLIDATED STATEMENTS OF STOCKHOLDERS' EQUITY - USD ($) $ in Thousands	Total	Common Stock	Additional Paid-In Capital	Retained Earnings	Accumulated Other Comprehensive Losses	Treasury Stock
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Balance at beginning of period (in shares)		57,564,493
Balance at beginning of period (in shares)		58,037,554
Balance at beginning of period at Dec. 31, 2017	$ 506,063	$ 737	$ 653,435	$ 98,728	$ (2,385)	$ (244,452)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income for the year	21,272			21,272
Foreign currency translation and other	409				409
Stock-based compensation (in shares)		380,588
Stock-based compensation	2,657	$ 6	2,651
Stock repurchase	(5,412)					(5,412)
Equity component of convertible senior notes repurchase	(7,830)		(7,830)
Common stock dividends	(4,748)			(4,748)
Stock option exercises (in shares)		92,473
Stock option exercises	861	$ 1	860
Balance at end of period (in shares) at Mar. 31, 2018		58,037,554
Balance at end of period at Mar. 31, 2018	513,272	$ 744	649,116	115,252	(1,976)	(249,864)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Balance at beginning of period (in shares)		57,267,116
Balance at beginning of period at Dec. 31, 2017	506,063	$ 737	653,435	98,728	(2,385)	(244,452)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income for the year	53,174
Foreign currency translation and other	(236)
Balance at end of period (in shares) at Jun. 30, 2018		57,267,116
Balance at end of period at Jun. 30, 2018	499,215	$ 744	624,209	142,747	(2,620)	(265,865)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Balance at beginning of period (in shares)		58,037,554
Balance at beginning of period (in shares)		57,267,116
Balance at beginning of period at Mar. 31, 2018	513,272	$ 744	649,116	115,252	(1,976)	(249,864)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income for the year	31,902			31,902
Foreign currency translation and other	(644)				(644)
Stock-based compensation (in shares)		24,040
Stock-based compensation	2,733	$ 0	2,733
Stock repurchase (in shares)		(798,992)
Stock repurchase	(16,001)					(16,001)
Equity component of convertible senior notes repurchase	(27,689)		(27,689)
Common stock dividends	(4,407)			(4,407)
Stock option exercises (in shares)		4,514
Stock option exercises	49	$ 0	49
Balance at end of period (in shares) at Jun. 30, 2018		57,267,116
Balance at end of period at Jun. 30, 2018	499,215	$ 744	624,209	142,747	(2,620)	(265,865)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Balance at beginning of period (in shares)		57,267,116
Balance at beginning of period (in shares)		55,135,788
Balance at beginning of period (in shares)		55,424,133
Balance at beginning of period at Dec. 31, 2018	473,849	$ 744	629,039	150,244	(3,343)	(302,835)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income for the year	14,780			14,780
Foreign currency translation and other	298				298
Stock-based compensation (in shares)		273,158
Stock-based compensation	2,586	$ 5	2,581
Stock repurchase	(2,635)					(2,635)
Common stock dividends	(4,512)			(4,512)
Unrealized loss on derivative instruments, net of tax	(814)				(814)
Stock option exercises (in shares)		15,187
Stock option exercises	54		54
Balance at end of period (in shares) at Mar. 31, 2019		55,424,133
Balance at end of period at Mar. 31, 2019	483,606	$ 749	631,674	160,512	(3,859)	(305,470)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Balance at beginning of period (in shares)		54,675,691
Balance at beginning of period at Dec. 31, 2018	473,849	$ 744	629,039	150,244	(3,343)	(302,835)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income for the year	45,740
Foreign currency translation and other	183
Balance at end of period (in shares) at Jun. 30, 2019		54,675,691
Balance at end of period at Jun. 30, 2019	501,565	$ 749	634,465	186,934	(3,896)	(316,687)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Balance at beginning of period (in shares)		55,424,133
Balance at beginning of period (in shares)		54,675,691
Balance at beginning of period at Mar. 31, 2019	483,606	$ 749	631,674	160,512	(3,859)	(305,470)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income for the year	30,960			30,960
Foreign currency translation and other	(115)				(115)
Stock-based compensation (in shares)		26,639
Stock-based compensation	2,791	$ 0	2,791
Stock repurchase (in shares)		(775,081)
Stock repurchase	(11,217)					(11,217)
Common stock dividends	(4,538)			(4,538)
Unrealized loss on derivative instruments, net of tax	78				78
Stock option exercises (in shares)
Stock option exercises	0
Balance at end of period (in shares) at Jun. 30, 2019		54,675,691
Balance at end of period at Jun. 30, 2019	$ 501,565	$ 749	$ 634,465	$ 186,934	$ (3,896)	$ (316,687)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Balance at beginning of period (in shares)		54,675,691