Note 11 - Shareholders' Equity - Schedule of Shareholders' Equity (Details) - USD ($) shares in Thousands, $ in Thousands	3 Months Ended						9 Months Ended
	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Sep. 30, 2019	Sep. 30, 2018
Balance	$ 1,113,524	$ 1,083,421	$ 1,066,928	$ 1,060,078	$ 1,031,669	$ 1,040,373	$ 1,066,928	$ 1,040,373
Stock options exercised and stock awards	904	1,745	1,231	630	1,184	766
Stock-based compensation related to stock options, restricted shares and employee stock purchase plan	2,709	4,258	8,836	2,008	5,949	7,893
Vesting of restricted share awards	0	(517)	(2,315)		(128)	(1,619)
Issuance of common stock under employee stock purchase plan	1,807	0	1,681	1,577		1,354
Common stock repurchases	(16,089)	(12,062)	(26,048)	(12,107)	(7,985)	(38,137)
Foreign currency translation adjustment	55	(505)	384
Net income	39,104	41,621	37,104	41,665	29,389	21,039	117,829	92,093
Balance	1,137,253	1,113,524	1,083,421	1,089,159	1,060,078	1,031,669	1,137,253	1,089,159
Common Class A [Member]
Dividend	(3,637)	(3,387)	(3,389)	(3,683)
Common Class B [Member]
Dividend	(1,124)	(1,050)	(991)	(1,009)
Common Stock Outstanding [Member]
Balance	462	462	458	457	457	454	458	454
Stock options exercised and stock awards	0	0	1	0	0	0
Stock-based compensation related to stock options, restricted shares and employee stock purchase plan	0	0	0	0	0	0
Vesting of restricted share awards	0	0	2		0	2
Issuance of common stock under employee stock purchase plan	1	0	1	1		1
Common stock repurchases	0	0	0	0	0	0
Dividend	0	0	0	0
Foreign currency translation adjustment	0	0	0
Net income	0	0	0	0	0	0
Balance	$ 463	$ 462	$ 462	$ 458	$ 457	$ 457	$ 463	$ 458
Common Stock Outstanding [Member] | Common Class A [Member]
Balance (in shares)	28,023	28,187	28,710	30,647	30,583	31,345	28,710	31,345
Stock options exercised and stock awards (in shares)	36	87	59	32	64	30
Vesting of restricted share awards (in shares)			0		0	1
Issuance of common stock under employee stock purchase plan (in shares)	60		57	43		41
Common stock repurchases (in shares)	(313)	(251)	(639)	220	0	(834)
Common stock repurchases (in shares)	313	251	639	(220)	(0)	834
Balance (in shares)	27,806	28,023	28,187	30,502	30,647	30,583	27,806	30,502
Common Stock Outstanding [Member] | Common Class B [Member]
Balance (in shares)	8,396	8,463	8,290	8,430	8,623	8,469	8,290	8,469
Stock options exercised and stock awards (in shares)	0	0	0	0	0	0
Vesting of restricted share awards (in shares)			226		1	226
Issuance of common stock under employee stock purchase plan (in shares)	0		0	0		0
Common stock repurchases (in shares)	(112)	(67)	(53)	71	(194)	(72)
Common stock repurchases (in shares)	112	67	53	(71)	194	72
Balance (in shares)	8,284	8,396	8,463	8,359	8,430	8,623	8,284	8,359
Additional Paid-in Capital [Member]
Balance	$ 384,940	$ 379,454	$ 370,025	$ 363,440	$ 356,435	$ 348,044	$ 370,025	$ 348,044
Stock options exercised and stock awards	904	1,745	1,230	630	1,184	766
Stock-based compensation related to stock options, restricted shares and employee stock purchase plan	2,709	4,258	8,836	2,008	5,949	7,893
Vesting of restricted share awards	0	(517)	(2,317)		(128)	(1,621)
Issuance of common stock under employee stock purchase plan	1,806	0	1,680	1,576		1,353
Common stock repurchases	0	0	0	0	0	0
Dividend	0	0	0	0
Foreign currency translation adjustment	0	0	0
Net income	0	0	0	0	0	0
Balance	390,359	384,940	379,454	367,654	363,440	356,435	390,359	367,654
Treasury Stock [Member]
Balance	(283,952)	(271,890)	(245,842)	(166,804)	(158,819)	(120,682)	(245,842)	(120,682)
Stock options exercised and stock awards	0	0	0	0	0	0
Stock-based compensation related to stock options, restricted shares and employee stock purchase plan	0	0	0	0	0	0
Vesting of restricted share awards	0	0	0		0	0
Issuance of common stock under employee stock purchase plan	0	0	0	0		0
Common stock repurchases	(16,089)	(12,062)	(26,048)	(12,107)	(7,985)	(38,137)
Dividend	0	0	0	0
Foreign currency translation adjustment	0	0	0
Net income	0	0	0	0	0	0
Balance	(300,041)	(283,952)	(271,890)	(178,911)	(166,804)	(158,819)	(300,041)	(178,911)
Retained Earnings [Member]
Balance	1,012,195	975,011	942,287	862,985	833,596	812,557	942,287	812,557
Stock options exercised and stock awards	0	0	0	0	0	0
Stock-based compensation related to stock options, restricted shares and employee stock purchase plan	0	0	0	0	0	0
Vesting of restricted share awards	0	0	0		0	0
Issuance of common stock under employee stock purchase plan	0	0	0	0		0
Common stock repurchases	0	0	0	0	0	0
Foreign currency translation adjustment	0	0	0
Net income	39,104	41,621	37,104	41,665	29,389	21,039
Balance	1,046,538	1,012,195	975,011	899,958	$ 862,985	$ 833,596	1,046,538	$ 899,958
Retained Earnings [Member] | Common Class A [Member]
Dividend	(3,637)	(3,387)	(3,389)	(3,683)
Retained Earnings [Member] | Common Class B [Member]
Dividend	(1,124)	(1,050)	(991)	$ (1,009)
AOCI Attributable to Parent [Member]
Balance	(121)	384	0				0
Stock options exercised and stock awards	0	0	0
Stock-based compensation related to stock options, restricted shares and employee stock purchase plan	0	0	0
Vesting of restricted share awards	0	0	0
Issuance of common stock under employee stock purchase plan	0	0	0
Common stock repurchases	0	0	0
Dividend	0	0	0
Foreign currency translation adjustment	55	(505)	384
Net income	0	0	0
Balance	$ (66)	$ (121)	$ 384				$ (66)