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, 2018	28,710			8,290
Balance at Dec. 31, 2018							$ 458	$ 370,025	$ (245,842)	$ 942,287	$ 0	$ 1,066,928
Stock options exercised and stock awards (in shares)	59			0
Stock options exercised and stock awards							1	1,230	0	0	0	1,231
Stock-based compensation related to stock options, restricted shares and employee stock purchase plan							0	8,836	0	0	0	8,836
Vesting of restricted share awards (in shares)	0			226
Vesting of restricted share awards							2	(2,317)	0	0	0	(2,315)
Issuance of common stock under employee stock purchase plan (in shares)	57			0
Issuance of common stock under employee stock purchase plan							1	1,680	0	0	0	1,681
Common stock repurchases (in shares)	(639)			(53)
Common stock repurchases							0	0	(26,048)	0	0	(26,048)
Dividends		$ (3,389)	$ (3,389)		$ (991)	$ (991)
Other comprehensive income							0	0	0	0	384	384
Net income							0	0	0	37,104	0	37,104
Balance (in shares) at Mar. 31, 2019	28,187			8,463
Balance at Mar. 31, 2019							462	379,454	(271,890)	975,011	384	1,083,421
Balance (in shares) at Dec. 31, 2019	27,954			8,240
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)	73			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			224
Vesting of restricted share awards							2	(2,416)	0	0	0	(2,414)
Issuance of common stock under employee stock purchase plan (in shares)	61			0
Issuance of common stock under employee stock purchase plan							1	1,900	0	0	0	1,901
Common stock repurchases (in shares)	(555)			(54)
Common stock repurchases							0	0	(19,902)	0	0	(19,902)
Dividends		$ (3,646)	$ (3,646)		$ (1,108)	$ (1,108)
Other comprehensive income							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	27,533			8,410
Balance at Mar. 31, 2020							$ 469	$ 406,725	$ (324,031)	$ 1,083,906	$ (1,593)	$ 1,165,476