Consolidated Statements of Shareholders' Equity - USD ($) $ 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, 2016	30,007,000			9,245,000
Balance at Dec. 31, 2016							$ 438	$ 309,127	$ (86,882)	$ 640,428	$ (286)	$ 862,825
Stock options exercised and stock awards (in shares)	1,219,000			0
Stock options exercised and stock awards							12	22,355	0	0	0	22,367
Stock-based compensation related to stock options, restricted shares and employee stock purchase plan							0	15,606	0	0	0	15,606
Vesting of restricted share awards (in shares)	7,000			199,000
Vesting of restricted share awards							3	(1,518)	0	0	0	(1,515)
Issuance of common stock under employee stock purchase plan (in shares)	112,000			0
Issuance of common stock under employee stock purchase plan							1	2,474	0	0	0	2,475
Common stock repurchases (in shares)	0			(975,000)
Common stock repurchases							0	0	(33,800)	0	0	(33,800)
Other comprehensive income							0	0	0	0	286	286
Net income							0	0	0	172,129	0	172,129
Balance (in shares) at Dec. 31, 2017	31,345,000			8,469,000
Balance at Dec. 31, 2017							454	348,044	(120,682)	812,557	0	1,040,373
Stock options exercised and stock awards (in shares)	138,000			0
Stock options exercised and stock awards							1	2,742	0	0	0	2,743
Stock-based compensation related to stock options, restricted shares and employee stock purchase plan							0	18,059	0	0	0	18,059
Vesting of restricted share awards (in shares)	0			227,000
Vesting of restricted share awards							2	(1,749)	0	0	0	(1,747)
Issuance of common stock under employee stock purchase plan (in shares)	84,000			0
Issuance of common stock under employee stock purchase plan							1	2,929	0	0	0	2,930
Common stock repurchases (in shares)	(2,857,000)			(406,000)
Common stock repurchases							0	0	(125,160)	0	0	(125,160)
Net income							0	0	0	139,062	0	139,062
Cash dividends declared on common stock		$ (7,324)	$ (7,324)		$ (2,008)	$ (2,008)
Balance (in shares) at Dec. 31, 2018	28,710,000			8,290,000
Balance at Dec. 31, 2018							458	370,025	(245,842)	942,287	0	$ 1,066,928
Stock options exercised and stock awards (in shares)	391,000			0								375,950
Stock options exercised and stock awards							4	7,585	0	0	0	$ 7,589
Stock-based compensation related to stock options, restricted shares and employee stock purchase plan							0	19,005	0	0	0	19,005
Vesting of restricted share awards (in shares)	0			226,000
Vesting of restricted share awards							2	(2,834)	0	0	0	(2,832)
Issuance of common stock under employee stock purchase plan (in shares)	117,000			0
Issuance of common stock under employee stock purchase plan							1	3,486	0	0	0	3,487
Common stock repurchases (in shares)	(1,264,000)			(276,000)
Common stock repurchases							0	0	(58,287)	0	0	(58,287)
Other comprehensive income							0	0	0	0	337	337
Net income							0	0	0	141,583	0	141,583
Cash dividends declared on common stock		$ (14,037)	$ (14,037)		$ (4,280)	$ (4,280)
Balance (in shares) at Dec. 31, 2019	27,954,000			8,240,000
Balance at Dec. 31, 2019							$ 465	$ 397,267	$ (304,129)	$ 1,065,553	$ 337	$ 1,159,493