Note 8 - Share Repurchases (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Common Class A [Member] | Share Repurchase Plan [Member]
Total purchase price (in thousands)	$ 24,676	$ 15,990	$ 5,213
Average purchase price per share (in dollars per share)	$ 107.88	$ 70.52	$ 40.76
Common Class A [Member]
Shares repurchased pursuant to repurchase plan (in shares)	228,737	226,729	127,900
Total purchase price (in thousands)	$ 31,548	$ 22,968	$ 7,903
Shares repurchased in association with tax withholdings on the exercise of stock options (in shares)	77,649	106,772	59,721