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, 2018		$ 268,000	$ 132,000	$ 506,970,000	$ (9,158,000)	$ 903,084,000	$ (102,000)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Stock-based compensation expense	$ 5,188,000	1,000		5,187,000
Stock option exercises		2,000		9,154,000
Payment consideration to tax authority on employees' behalf				(1,476,000)
Net income	73,269,000					73,269,000
Dividends						(32,360,000)
Translation adjustments, net of tax	9,000						9,000
Cumulative-effect adjustment for adoption of Accounting Standards Update 2018-02	(24,000)						(24,000)
At end of period at Sep. 30, 2019	1,454,956,000	271,000	132,000	519,835,000	(9,158,000)	943,993,000	(117,000)
At beginning of period at Jun. 30, 2019		270,000	132,000	513,840,000	(9,158,000)	926,879,000	(121,000)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Stock-based compensation expense		0		1,938,000
Stock option exercises		1,000		4,176,000
Payment consideration to tax authority on employees' behalf				(119,000)
Net income	27,937,000					27,937,000
Dividends						(10,823,000)
Translation adjustments, net of tax	4,000						4,000
Cumulative-effect adjustment for adoption of Accounting Standards Update 2018-02	0						0
At end of period at Sep. 30, 2019	1,454,956,000	271,000	132,000	519,835,000	(9,158,000)	943,993,000	(117,000)
At beginning of period at Dec. 31, 2019	1,490,642,000	272,000	132,000	525,851,000	(9,158,000)	973,767,000	(222,000)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Stock-based compensation expense	8,533,000	1,000		8,532,000
Stock option exercises		1,000		5,917,000
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)
Cumulative-effect adjustment for adoption of Accounting Standards Update 2018-02	0						0
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-based compensation expense		0		3,023,000
Stock option exercises		0		1,036,000
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)
Cumulative-effect adjustment for adoption of Accounting Standards Update 2018-02	0						0
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)