Consolidated Statements of Shareholders' Equity (Unaudited) - USD ($) shares in Thousands, $ in Thousands	Common Class A [Member] Common Stock [Member]	Common Class A [Member] Retained Earnings [Member]	Common Class A [Member]	Common Class B [Member] Common Stock [Member]	Common Class B [Member] Additional Paid-in Capital [Member]	Common Class B [Member] Treasury Stock [Member]	Common Class B [Member] Retained Earnings [Member]	Common Class B [Member] AOCI Attributable to Parent [Member]	Common Class B [Member]	Common Stock [Member]	Additional Paid-in Capital [Member]	Treasury Stock [Member]	Retained Earnings [Member]	AOCI Attributable to Parent [Member]	Total
Balance (in shares) at Dec. 31, 2019	41,930			12,360
Balance at Dec. 31, 2019										$ 465	$ 397,267	$ (304,129)	$ 1,065,553	$ 337	$ 1,159,493
Stock options exercised and stock awards (in shares)	110			0
Stock options exercised and stock awards										1	1,421	0	0	0	1,422
Stock-based compensation related to stock options, restricted shares and employee stock purchase plan										0	8,553	0	0	0	8,553
Vesting of restricted share awards (in shares)	0			337
Vesting of restricted share awards										2	(2,416)	0	0	0	(2,414)
Issuance of common stock under employee stock purchase plan (in shares)	92			0
Issuance of common stock under employee stock purchase plan										1	1,900	0	0	0	1,901
Common stock repurchases (in shares)	(833)			(81)
Common stock repurchases										0	0	(19,902)	0	0	(19,902)
Dividend common stock		$ (3,646)	$ (3,646)				$ (1,108)		$ (1,108)
Other comprehensive income (loss)										0	0	0	0	(1,930)	(1,930)
Net income										0	0	0	23,107	0	23,107
Dividend common stock		(3,646)	(3,646)				(1,108)		(1,108)
Balance (in shares) at Mar. 31, 2020	41,299			12,616
Balance at Mar. 31, 2020										469	406,725	(324,031)	1,083,906	(1,593)	1,165,476
Balance (in shares) at Dec. 31, 2019	41,930			12,360
Balance at Dec. 31, 2019										465	397,267	(304,129)	1,065,553	337	1,159,493
Net income															39,923
Balance (in shares) at Jun. 30, 2020	41,593			12,616
Balance at Jun. 30, 2020										471	414,812	(324,031)	1,095,976	(668)	1,186,560
Balance (in shares) at Mar. 31, 2020	41,299			12,616
Balance at Mar. 31, 2020										469	406,725	(324,031)	1,083,906	(1,593)	1,165,476
Stock options exercised and stock awards (in shares)	294			0
Stock options exercised and stock awards										2	4,529	0	0	0	4,531
Stock-based compensation related to stock options, restricted shares and employee stock purchase plan										0	3,586	0	0	0	3,586
Vesting of restricted share awards										0	(28)	0	0	0	(28)
Dividend common stock		(3,581)	(3,581)	$ 0	$ 0	$ 0	(1,165)	$ 0	(1,165)
Other comprehensive income (loss)										0	0	0	0	925	925
Net income										0	0	0	16,816	0	16,816
Dividend common stock		(3,581)	(3,581)	$ 0	$ 0	$ 0	(1,165)	$ 0	(1,165)
Balance (in shares) at Jun. 30, 2020	41,593			12,616
Balance at Jun. 30, 2020										471	414,812	(324,031)	1,095,976	(668)	1,186,560
Balance (in shares) at Dec. 31, 2020	42,504			12,470
Balance at Dec. 31, 2020										551	437,646	(2,879)	831,850	869	1,268,037
Stock options exercised and stock awards (in shares)	298			0
Stock options exercised and stock awards										3	5,416	0	0	0	5,419
Stock-based compensation related to stock options, restricted shares and employee stock purchase plan										0	11,520	0	0	0	11,520
Vesting of restricted share awards (in shares)	0			345
Vesting of restricted share awards										3	(6,780)	0	0	0	(6,777)
Issuance of common stock under employee stock purchase plan (in shares)	86			0
Issuance of common stock under employee stock purchase plan										1	1,988	0	0	0	1,989
Common stock repurchases (in shares)	(3)			(155)
Common stock repurchases										0	0	(6,483)	0	0	(6,483)
Dividend common stock		(7,684)	(7,684)				(2,380)		(2,380)
Other comprehensive income (loss)										0	0	0	0	255	255
Net income										0	0	0	45,333	0	45,333
Dividend common stock		(7,684)	(7,684)				(2,380)		(2,380)
Balance (in shares) at Mar. 31, 2021	42,885			12,660
Balance at Mar. 31, 2021										558	449,790	(9,362)	867,119	1,124	1,309,229
Balance (in shares) at Dec. 31, 2020	42,504			12,470
Balance at Dec. 31, 2020										551	437,646	(2,879)	831,850	869	1,268,037
Net income															103,377
Balance (in shares) at Jun. 30, 2021	43,104			12,561
Balance at Jun. 30, 2021										560	456,785	(13,585)	915,003	1,436	1,360,199
Balance (in shares) at Mar. 31, 2021	42,885			12,660
Balance at Mar. 31, 2021										558	449,790	(9,362)	867,119	1,124	1,309,229
Stock options exercised and stock awards (in shares)	219			0
Stock options exercised and stock awards										2	3,926	0	0	0	3,928
Stock-based compensation related to stock options, restricted shares and employee stock purchase plan										0	3,683	0	0	0	3,683
Vesting of restricted share awards										0	(614)	0	0	0	(614)
Common stock repurchases (in shares)	0			(99)
Common stock repurchases										0	0	(4,223)	0	0	(4,223)
Dividend common stock		(7,735)	(7,735)				(2,425)		(2,425)
Other comprehensive income (loss)										0	0	0	0	312	312
Net income										0	0	0	58,044	0	58,044
Dividend common stock		$ (7,735)	$ (7,735)				$ (2,425)		$ (2,425)
Balance (in shares) at Jun. 30, 2021	43,104			12,561
Balance at Jun. 30, 2021										$ 560	$ 456,785	$ (13,585)	$ 915,003	$ 1,436	$ 1,360,199