Note 17 - Net Income Per Share of Class A and Class B Common Stock - Earnings Per Share Reconciliation (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Shares used in basic per share calculations (in shares)	25,409	26,290	26,121
Shares used in diluted per share calculations (in shares)	25,521	26,490	26,382
Basic net income per share (in dollars per share)	$ 7.76	$ 6.96	$ 5.31
Diluted net income per share (in dollars per share)	$ 7.72	$ 6.91	$ 5.26
Common Class A [Member]
Net income applicable to common stockholders - basic	$ 182,369	$ 165,172	$ 122,246
Reallocation of distributed net income as a result of conversion of dilutive stock options	8	15	15
Reallocation of distributed net income due to conversion of Class B to Class A common shares outstanding	1,791	1,932	1,547
Conversion of Class B common shares into Class A common shares	12,833	15,760	11,616
Effect of dilutive stock options on net income	57	120	116
Net income applicable to common stockholders - diluted	$ 197,058	$ 182,999	$ 135,540
Shares used in basic per share calculations (in shares)	23,515	23,729	23,559
Conversion of Class B common shares into Class A common shares (in shares)	1,894	2,561	2,562
Effect of dilutive stock options on weighted average common shares (in shares)	112	200	261
Shares used in diluted per share calculations (in shares)	25,521	26,490	26,382
Basic net income per share (in dollars per share)	$ 7.76	$ 6.96	$ 5.19
Diluted net income per share (in dollars per share)	$ 7.72	$ 6.91	$ 5.14
Common Class A [Member] | Employee Stock Option [Member]
Shares issuable pursuant to stock options not included since they were antidilutive (in shares)		16	13
Common Class B [Member]
Net income applicable to common stockholders - basic	$ 14,689	$ 17,827	$ 13,294
Reallocation of distributed net income as a result of conversion of dilutive stock options	(8)	(15)	(15)
Reallocation of distributed net income due to conversion of Class B to Class A common shares outstanding
Conversion of Class B common shares into Class A common shares
Effect of dilutive stock options on net income	(57)	(120)	(116)
Net income applicable to common stockholders - diluted	$ 14,624	$ 17,692	$ 13,163
Shares used in basic per share calculations (in shares)	1,894	2,561	2,562
Conversion of Class B common shares into Class A common shares (in shares)
Effect of dilutive stock options on weighted average common shares (in shares)
Shares used in diluted per share calculations (in shares)	1,894	2,561	2,562
Basic net income per share (in dollars per share)	$ 7.76	$ 6.96	$ 5.19
Diluted net income per share (in dollars per share)	$ 7.72	$ 6.91	$ 5.14
Common Class B [Member] | Employee Stock Option [Member]
Shares issuable pursuant to stock options not included since they were antidilutive (in shares)