Consolidated Statements of Shareholders' Equity - USD ($) $ in Thousands	Common Class A [Member] Common Stock [Member]	Common Class A [Member] Additional Paid-in Capital [Member]	Common Class A [Member] Treasury Stock [Member]	Common Class A [Member] Retained Earnings [Member]	Common Class A [Member] AOCI Attributable to Parent [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, 2017	47,017,000						12,703,000
Balance at Dec. 31, 2017													$ 454	$ 348,044	$ (120,682)	$ 812,557	$ 0	$ 1,040,373
Stock options exercised and stock awards (in shares)	207,000						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						340,000
Vesting of restricted share awards													2	(1,749)	0	0	0	(1,747)
Issuance of common stock under employee stock purchase plan (in shares)	126,000						0
Issuance of common stock under employee stock purchase plan													1	2,929	0	0	0	2,930
Common stock repurchases (in shares)	(4,285,000)						(608,000)
Common stock repurchases													0	0	(125,160)	0	0	(125,160)
Cash dividends declared	$ 0	$ 0	$ 0	$ (7,324)	$ 0	$ (7,324)	$ 0	$ 0	$ 0	$ (2,008)	$ 0	$ (2,008)
Net income													0	0	0	139,062	0	139,062
Balance (in shares) at Dec. 31, 2018	43,065,000						12,435,000
Balance at Dec. 31, 2018													458	370,025	(245,842)	942,287	0	1,066,928
Stock options exercised and stock awards (in shares)	586,000						0
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													2	(2,834)	0	0	0	(2,832)
Issuance of common stock under employee stock purchase plan (in shares)	175,000						0
Issuance of common stock under employee stock purchase plan													1	3,486	0	0	0	3,487
Common stock repurchases (in shares)	(1,896,000)						(414,000)
Common stock repurchases													0	0	(58,287)	0	0	(58,287)
Cash dividends declared	$ 0	0	0	(14,037)	0	(14,037)	$ 0	0	0	(4,280)	0	(4,280)
Net income													0	0	0	141,583	0	141,583
Other comprehensive income (loss)													0	0	0	0	337	337
Balance (in shares) at Dec. 31, 2019	41,930,000						12,360,000
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)	1,247,000						0											1,221,594
Stock options exercised and stock awards													10	19,582	0	0	0	$ 19,592
Stock-based compensation related to stock options, restricted shares and employee stock purchase plan													0	19,356	0	0	0	19,356
Vesting of restricted share awards (in shares)	0						339,000
Vesting of restricted share awards													2	(2,459)	0	0	0	(2,457)
Issuance of common stock under employee stock purchase plan (in shares)	177,000						0
Issuance of common stock under employee stock purchase plan													2	3,900	0	0	0	3,902
Common stock repurchases (in shares)	(843,000)						(225,000)
Common stock repurchases													0	0	(24,807)	0	0	(24,807)
Cash dividends declared	$ 0	$ 0	$ 0	$ (17,062)	$ 0	$ (17,062)	$ 0	$ 0	$ 0	$ (5,399)	$ 0	$ (5,399)
Net income													0	0	0	114,887	0	114,887
Other comprehensive income (loss)													0	0	0	0	532	532
Cancellation of treasury stock (in shares)	(7,000)						(4,000)
Cancellation of treasury stock													72	0	326,057	(326,129)	0
Balance (in shares) at Dec. 31, 2020	42,504,000						12,470,000
Balance at Dec. 31, 2020													$ 551	$ 437,646	$ (2,879)	$ 831,850	$ 869	$ 1,268,037