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		4,881,000
Stock-based compensation expense	$ 5,510,000	1,000		5,509,000
Payment consideration to tax authority on employees' behalf				(777,000)
Net income	58,627,000					58,627,000
Dividends						(25,770,000)
Translation adjustments, net of tax	(25,000)						(25,000)
At end of period at Jun. 30, 2020	1,533,089,000	274,000	132,000	535,464,000	(9,158,000)	1,006,624,000	(247,000)
At beginning of period at Mar. 31, 2020		273,000	132,000	529,763,000	(9,158,000)	989,578,000	(239,000)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Stock option exercises		0		2,827,000
Stock-based compensation expense		1,000		2,874,000
Payment consideration to tax authority on employees' behalf				0
Net income	29,948,000					29,948,000
Dividends						(12,902,000)
Translation adjustments, net of tax	(8,000)						(8,000)
At end of period at Jun. 30, 2020	1,533,089,000	274,000	132,000	535,464,000	(9,158,000)	1,006,624,000	(247,000)
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		6,432,000
Stock-based compensation expense	7,556,000	1,000		7,555,000
Payment consideration to tax authority on employees' behalf				(2,493,000)
Net income	68,937,000					68,937,000
Dividends						(30,851,000)
Translation adjustments, net of tax	(19,000)						(19,000)
At end of period at Jun. 30, 2021	1,628,976,000	278,000	132,000	557,211,000	(9,158,000)	1,080,762,000	(249,000)
At beginning of period at Mar. 31, 2021		277,000	132,000	549,811,000	(9,158,000)	1,059,608,000	(242,000)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Stock option exercises		1,000		3,291,000
Stock-based compensation expense		0		4,113,000
Payment consideration to tax authority on employees' behalf				(4,000)
Net income	36,609,000					36,609,000
Dividends						(15,455,000)
Translation adjustments, net of tax	(7,000)						(7,000)
At end of period at Jun. 30, 2021	$ 1,628,976,000	$ 278,000	$ 132,000	$ 557,211,000	$ (9,158,000)	$ 1,080,762,000	$ (249,000)