Note 8 - Stockholders' Equity - Share Repurchases (Details) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Shares repurchased in association with tax withholdings on the vesting of RSUs (in shares)	94,826	77,649	106,772
Common Class A [Member]
Shares repurchased pursuant to repurchase authorizations (in shares)	1,312,848	228,737	226,729
Average purchase price per share (in dollars per share)	$ 79.50	$ 107.88	$ 70.52
Total purchase price (in thousands)	$ 104,370	$ 24,676	$ 15,990
The 2011 Stock Repurchase Authorization [Member] | Common Class A [Member]
Shares repurchased pursuant to repurchase authorizations (in shares)	599,123
Average purchase price per share (in dollars per share)	$ 79.21
Cumulative repurchases, shares (in shares)	2,327,636
Cumulative repurchases, average price (in dollars per share)	$ 51.09
The 2016 Stock Repurchase Authorization [Member] | Common Class A [Member]
Shares repurchased pursuant to repurchase authorizations (in shares)	713,725
Average purchase price per share (in dollars per share)	$ 79.74
Cumulative repurchases, shares (in shares)	713,725
Cumulative repurchases, average price (in dollars per share)	$ 79.74