Note 5 - Stockholders' Equity (Details Textual) $ / shares in Units, $ in Thousands	6 Months Ended
	Jun. 30, 2018 USD ($) $ / shares shares	Jun. 30, 2017 USD ($)	Dec. 31, 2017 shares	Nov. 06, 2016 USD ($)	Mar. 31, 2006 shares	Nov. 30, 2004 shares	Mar. 31, 2004 shares
Capital Units, Authorized	245,000,000
Common Stock, Shares Authorized	200,000,000		100,000,000
Preferred Stock, Shares Authorized	20,000,000
Common Stock, Shares, Outstanding, Ending Balance	82,022,500		83,253,588
Preferred Stock, Shares Outstanding, Ending Balance	0
Common Stock, Voting Rights, Votes Per Share	1
Dividends, Common Stock, Total | $	$ 0	$ 0
Treasury Stock, Value, Acquired, Cost Method | $	$ 19,607
Common Stock, Capital Shares Reserved for Future Issuance	7,078,916
March 2004 Authorization [Member]
Stock Repurchase Program, Number of Shares Authorized to be Repurchased							2,000,000
The 2004-2006 Repurchase Authorization [Member]
Stock Repurchase Program, Number of Shares Authorized to be Repurchased					5,000,000
Stock Repurchase Program, Remaining Number of Shares Authorized to be Repurchased	279,200
The 2016 Repurchase Authorization [Member]
Stock Repurchase Program, Authorized Amount | $				$ 75,000
Treasury Stock, Shares, Acquired	1,551,710
Treasury Stock Acquired, Average Cost Per Share | $ / shares	$ 12.64
Treasury Stock, Value, Acquired, Cost Method | $	$ 19,600
Stock Repurchase Program, Remaining Authorized Repurchase Amount | $	$ 49,500
November 2004 Authorization [Member]
Stock Repurchase Program, Number of Shares Authorized to be Repurchased						2,000,000
Common Class A [Member]
Common Stock, Shares Authorized	25,000,000		15,000,000
Common Stock, Shares, Outstanding, Ending Balance	6,729,035		6,598,377
Common Stock, Voting Rights, Votes Per Share	10
Common Stock, Capital Shares Reserved for Future Issuance	1,703,064