Consolidated Statements of Shareholders' Equity (Unaudited) - USD ($) shares in Thousands, $ 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, 2018	43,064						12,435
Balance at Dec. 31, 2018													$ 458	$ 370,025	$ (245,842)	$ 942,287	$ 0	$ 1,066,928
Stock options exercised and stock awards (in shares)	88						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)	1						339
Vesting of restricted share awards													2	(2,317)	0	0	0	(2,315)
Issuance of common stock under employee stock purchase plan (in shares)	86						0
Issuance of common stock under employee stock purchase plan													1	1,680	0	0	0	1,681
Common stock repurchases (in shares)	(959)						(80)
Common stock repurchases													0	0	(26,048)	0	0	(26,048)
Dividends				$ (3,389)		$ (3,389)				$ (991)		$ (991)
Other comprehensive income (loss)													0	0	0	0	384	384
Net income													0	0	0	37,104	0	37,104
Balance (in shares) at Mar. 31, 2019	42,280						12,694
Balance at Mar. 31, 2019													462	379,454	(271,890)	975,011	384	1,083,421
Balance (in shares) at Dec. 31, 2018	43,064						12,435
Balance at Dec. 31, 2018													458	370,025	(245,842)	942,287	0	1,066,928
Net income																		117,829
Balance (in shares) at Sep. 30, 2019	41,710						12,426
Balance at Sep. 30, 2019													463	390,359	(300,041)	1,046,538	(66)	1,137,253
Balance (in shares) at Mar. 31, 2019	42,280						12,694
Balance at Mar. 31, 2019													462	379,454	(271,890)	975,011	384	1,083,421
Stock options exercised and stock awards (in shares)	131						0
Stock options exercised and stock awards													0	1,745	0	0	0	1,745
Stock-based compensation related to stock options, restricted shares and employee stock purchase plan													0	4,258	0	0	0	4,258
Vesting of restricted share awards													0	(517)	0	0	0	(517)
Common stock repurchases (in shares)	(377)						(100)
Common stock repurchases													0	0	(12,062)	0	0	(12,062)
Dividends				(3,387)		(3,387)				(1,050)		(1,050)
Other comprehensive income (loss)													0	0	0	0	(505)	(505)
Net income													0	0	0	41,621	0	41,621
Balance (in shares) at Jun. 30, 2019	42,034						12,594
Balance at Jun. 30, 2019													462	384,940	(283,952)	1,012,195	(121)	1,113,524
Stock options exercised and stock awards (in shares)	55						0
Stock options exercised and stock awards													0	904	0	0	0	904
Stock-based compensation related to stock options, restricted shares and employee stock purchase plan													0	2,709	0	0	0	2,709
Vesting of restricted share awards													0	0	0	0	0	0
Issuance of common stock under employee stock purchase plan (in shares)	90						0
Issuance of common stock under employee stock purchase plan													1	1,806	0	0	0	1,807
Common stock repurchases (in shares)	(469)						(168)
Common stock repurchases													0	0	(16,089)	0	0	(16,089)
Dividends	$ 0	$ 0	$ 0	(3,637)	$ 0	(3,637)	$ 0	$ 0	$ 0	(1,124)	$ 0	(1,124)
Other comprehensive income (loss)													0	0	0	0	55	55
Net income													0	0	0	39,104	0	39,104
Balance (in shares) at Sep. 30, 2019	41,710						12,426
Balance at Sep. 30, 2019													463	390,359	(300,041)	1,046,538	(66)	1,137,253
Balance (in shares) at Dec. 31, 2019	41,930						12,361
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)						(82)
Common stock repurchases													0	0	(19,902)	0	0	(19,902)
Dividends				(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, 2019	41,930						12,361
Balance at Dec. 31, 2019													465	397,267	(304,129)	1,065,553	337	1,159,493
Net income																		73,862
Balance (in shares) at Sep. 30, 2020	42,208						12,519
Balance at Sep. 30, 2020													547	428,823	(723)	798,606	(180)	1,227,073
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)
Dividends				(3,581)		(3,581)				(1,165)		(1,165)
Other comprehensive income (loss)													0	0	0	0	925	925
Net income													0	0	0	16,816	0	16,816
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
Stock options exercised and stock awards (in shares)	537						0
Stock options exercised and stock awards													3	8,701	0	0	0	8,704
Stock-based compensation related to stock options, restricted shares and employee stock purchase plan													0	3,324	0	0	0	3,324
Vesting of restricted share awards (in shares)	0						2
Vesting of restricted share awards													0	(14)	0	0	0	(14)
Issuance of common stock under employee stock purchase plan (in shares)	85						0
Issuance of common stock under employee stock purchase plan													1	2,000				2,001
Common stock repurchases (in shares)							(95)
Common stock repurchases															(2,749)			(2,749)
Dividends	$ 0	$ 0	$ 0	$ (3,922)	$ 0	$ (3,922)	$ 0	$ 0	$ 0	$ (1,258)	$ 0	$ (1,258)
Other comprehensive income (loss)													0	0	0	0	488	488
Net income													0	0	0	33,939	0	33,939
Cancellation of treasury stock (in shares)	(7)						(4)
Cancellation of treasury stock													72		326,057	(326,129)		0
Balance (in shares) at Sep. 30, 2020	42,208						12,519
Balance at Sep. 30, 2020													$ 547	$ 428,823	$ (723)	$ 798,606	$ (180)	$ 1,227,073