CONSOLIDATED STATEMENTS OF STOCKHOLDERS' EQUITY - USD ($) shares in Thousands, $ in Thousands	Total	Class A	Class B Convertible	Additional Paid-in Capital	Treasury Stock	Accumulated Other Comprehensive Income (Loss)	Retained Earnings
Beginning Balance at Dec. 31, 2018	$ 529,731	$ 16	$ 2	$ 576,957	$ (586,161)	$ (10,217)	$ 549,134
Beginning Balance (in shares) at Dec. 31, 2018		15,837	2,035		(7,285)
Net income (loss)	(7,906)	$ 0	$ 0	0	$ 0	0	(7,906)
Other comprehensive income (loss)	253	0	0	0	0	253	0
Issuance of class A common stock under stock option plans	1,507	$ 0	$ 0	1,507	$ 0	0	0
Issuance of class A common stock under stock option plans (in shares)		13	0		0
Purchases of treasury stock	(48,244)	$ 0	$ 0	0	$ (48,244)	0	0
Purchases of treasury stock (in shares)		0	0		(363)
Share-based compensation expense	2,965	$ 0	$ 0	2,965	$ 0	0	0
Ending Balance at Mar. 31, 2019	478,306	$ 16	$ 2	581,429	$ (634,405)	(9,964)	541,228
Ending Balance (in shares) at Mar. 31, 2019		15,850	2,035		(7,648)
Net income (loss)	20,394	$ 0	$ 0	0	$ 0	0	20,394
Other comprehensive income (loss)	654	0	0	0	0	654	0
Issuance of class A common stock under stock option plans	329	$ 0	$ 0	329	$ 0	0	0
Issuance of class A common stock under stock option plans (in shares)		3	0		0
Share-based compensation expense	3,006	$ 0	$ 0	3,006	$ 0	0	0
Ending Balance at Jun. 30, 2019	502,689	$ 16	$ 2	584,764	$ (634,405)	(9,310)	561,622
Ending Balance (in shares) at Jun. 30, 2019		15,853	2,035		(7,648)
Net income (loss)	9,700	$ 0	$ 0	0	$ 0	0	9,700
Other comprehensive income (loss)	(2,364)	0	0	0	0	(2,364)	0
Issuance of class A common stock under stock option plans	3,606	$ 0	$ 0	3,606	$ 0	0	0
Issuance of class A common stock under stock option plans (in shares)		27	0		0
Share-based compensation expense	1,788	$ 0	$ 0	1,788	$ 0	0	0
Ending Balance at Sep. 30, 2019	515,419	$ 16	$ 2	590,158	$ (634,405)	(11,674)	571,322
Ending Balance (in shares) at Sep. 30, 2019		15,880	2,035		(7,648)
Net income (loss)	12,167	$ 0	$ 0	0	$ 0	0	12,167
Other comprehensive income (loss)	2,023	0	0	0	0	2,023	0
Issuance of class A common stock under stock option plans	1,127	$ 0	$ 0	1,127	$ 0	0	0
Issuance of class A common stock under stock option plans (in shares)		8	0		0
Purchases of treasury stock	(24,475)	$ 0	$ 0	0	$ (24,475)	0	0
Purchases of treasury stock (in shares)		0	0		(159)
Share-based compensation expense	2,298	$ 0	$ 0	2,298	$ 0	0	0
Ending Balance at Dec. 31, 2019	508,559	$ 16	$ 2	593,583	$ (658,880)	(9,651)	583,489
Ending Balance (in shares) at Dec. 31, 2019		15,888	2,035		(7,807)
Net income (loss)	657	$ 0	$ 0	0	$ 0	0	657
Other comprehensive income (loss)	(547)	0	0	0	0	(547)	0
Issuance of class A common stock under stock option plans	340	$ 0	$ 0	340	$ 0	0	0
Issuance of class A common stock under stock option plans (in shares)		3	0		0
Purchases of treasury stock	(50,747)	$ 0	$ 0	0	$ (50,747)	0	0
Purchases of treasury stock (in shares)		0	0		(355)
Share-based compensation expense	3,164	$ 0	$ 0	3,164	$ 0	0	0
Ending Balance at Mar. 31, 2020	$ 461,426	$ 16	$ 2	$ 597,087	$ (709,627)	$ (10,198)	$ 584,146
Ending Balance (in shares) at Mar. 31, 2020		15,891	2,035		(8,162)