Note 11 - Shareholders' Equity - Schedule of Shareholders' Equity (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Balance			$ 1,040,373
Stock options exercised and stock awards			2,581
Stock-based compensation related to stock options, restricted shares and employee stock purchase plan			15,850
Vesting of restricted share awards			(1,747)
Issuance of common stock under employee stock purchase plan			2,930
Common stock repurchases			(58,229)
Net income	$ 41,665	$ 29,784	92,093	$ 66,262
Balance	1,089,159		1,089,159
Common Class A [Member]
Dividend			(3,683)
Common Class B [Member]
Dividend			(1,009)
Common Stock [Member]
Balance			454
Stock options exercised and stock awards			1
Vesting of restricted share awards			2
Issuance of common stock under employee stock purchase plan			1
Balance	$ 458		$ 458
Common Stock [Member] | Common Class A [Member]
Balance (in shares)			31,345
Stock options exercised and stock awards (in shares)			127
Vesting of restricted share awards (in shares)
Issuance of common stock under employee stock purchase plan (in shares)			84
Common stock repurchases (in shares)			(1,054)
Balance (in shares)	30,502		30,502
Common Stock [Member] | Common Class B [Member]
Balance (in shares)			8,469
Stock options exercised and stock awards (in shares)
Vesting of restricted share awards (in shares)			227
Issuance of common stock under employee stock purchase plan (in shares)
Common stock repurchases (in shares)			(337)
Balance (in shares)	8,359		8,359
Additional Paid-in Capital [Member]
Balance			$ 348,044
Stock options exercised and stock awards			2,580
Stock-based compensation related to stock options, restricted shares and employee stock purchase plan			15,850
Vesting of restricted share awards			(1,749)
Issuance of common stock under employee stock purchase plan			2,929
Balance	$ 367,654		367,654
Treasury Stock [Member]
Balance			(120,682)
Common stock repurchases			(58,229)
Balance	(178,911)		(178,911)
Retained Earnings [Member]
Balance			812,557
Net income			92,093
Balance	$ 899,958		899,958
Retained Earnings [Member] | Common Class A [Member]
Dividend			(3,683)
Retained Earnings [Member] | Common Class B [Member]
Dividend			$ (1,009)