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] Retained Earnings [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
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, 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
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