Condensed Consolidated Statements of Changes in Stockholders' Equity Statement - USD ($)	Total	Common Stock, Class A	Common Stock, Class B	Additional Paid-In Capital	Treasury Stock, at cost	Retained Earnings	Accumulated Other Comprehensive Loss
At beginning of period at Dec. 31, 2019		$ 272,000	$ 132,000	$ 525,851,000	$ (9,158,000)	$ 973,767,000	$ (222,000)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Stock option exercises		1,000		2,054,000
Stock-based compensation expense	$ 2,635,000	0		2,635,000
Payment consideration to tax authority on employees' behalf				(777,000)
Net income	28,679,000					28,679,000
Dividends						(12,868,000)
Translation adjustments, net of tax	(17,000)						(17,000)
At end of period at Mar. 31, 2020	1,510,349,000	273,000	132,000	529,763,000	(9,158,000)	989,578,000	(239,000)
At beginning of period at Dec. 31, 2020	1,579,412,000	275,000	132,000	545,717,000	(9,158,000)	1,042,676,000	(230,000)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Stock option exercises		1,000		3,141,000
Stock-based compensation expense	3,443,000	1,000		3,442,000
Payment consideration to tax authority on employees' behalf				(2,489,000)
Net income	32,328,000					32,328,000
Dividends						(15,396,000)
Translation adjustments, net of tax	(12,000)						(12,000)
At end of period at Mar. 31, 2021	$ 1,600,428,000	$ 277,000	$ 132,000	$ 549,811,000	$ (9,158,000)	$ 1,059,608,000	$ (242,000)