Condensed Consolidated Statements of Changes in Stockholders' Equity Statement - USD ($)	Total	Common Stock Common Stock, Class A	Common Stock 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		5,917,000
Stock-based compensation expense	$ 8,533,000	1,000		8,532,000
Payment consideration to tax authority on employees' behalf				(1,014,000)
Net income	88,366,000					88,366,000
Dividends						(38,685,000)
Translation adjustments, net of tax	(44,000)						(44,000)
At end of period at Sep. 30, 2020	1,553,716,000	274,000	132,000	539,286,000	(9,158,000)	1,023,448,000	(266,000)
At beginning of period at Jun. 30, 2020		274,000	132,000	535,464,000	(9,158,000)	1,006,624,000	(247,000)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Stock option exercises		0		1,036,000
Stock-based compensation expense		0		3,023,000
Payment consideration to tax authority on employees' behalf				(237,000)
Net income	29,739,000					29,739,000
Dividends						(12,915,000)
Translation adjustments, net of tax	(19,000)						(19,000)
At end of period at Sep. 30, 2020	1,553,716,000	274,000	132,000	539,286,000	(9,158,000)	1,023,448,000	(266,000)
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		2,000		8,019,000
Stock-based compensation expense	11,516,000	1,000		11,515,000
Payment consideration to tax authority on employees' behalf				(3,092,000)
Net income	106,893,000					106,893,000
Dividends						(46,326,000)
Translation adjustments, net of tax	(22,000)						(22,000)
At end of period at Sep. 30, 2021	1,656,402,000	278,000	132,000	562,159,000	(9,158,000)	1,103,243,000	(252,000)
At beginning of period at Jun. 30, 2021		278,000	132,000	557,211,000	(9,158,000)	1,080,762,000	(249,000)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Stock option exercises		0		1,587,000
Stock-based compensation expense		0		3,960,000
Payment consideration to tax authority on employees' behalf				(599,000)
Net income	37,956,000					37,956,000
Dividends						(15,475,000)
Translation adjustments, net of tax	(3,000)						(3,000)
At end of period at Sep. 30, 2021	$ 1,656,402,000	$ 278,000	$ 132,000	$ 562,159,000	$ (9,158,000)	$ 1,103,243,000	$ (252,000)