Note 5 - Stockholders' Equity (Details Textual) $ / shares in Units, $ in Thousands	1 Months Ended		12 Months Ended
	Dec. 31, 2017 USD ($) $ / shares shares	Mar. 31, 2015 USD ($) $ / shares shares	Dec. 31, 2017 USD ($) $ / shares shares	Dec. 31, 2016 USD ($) shares	Dec. 31, 2015 USD ($)	Nov. 06, 2016 USD ($)	Mar. 31, 2006 shares	Nov. 30, 2004 shares	Mar. 31, 2004 shares
Capital Units, Authorized	135,000,000		135,000,000
Common Stock, Shares Authorized	100,000,000		100,000,000	100,000,000
Common Stock, Voting Rights, Votes Per Share	1		1
Dividends, Common Stock | $			$ 0	$ 0	$ 0
Proceeds from Issuance of Common Stock | $			238,945	0	167,313
Treasury Stock, Value, Acquired, Cost Method | $			$ 4,000	$ 2,000
Common Stock, Capital Shares Reserved for Future Issuance	7,422,965		7,422,965	5,449,148
Payments of Dividends | $			$ 0	$ 0	$ 0
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		279,200
The 2016 Repurchase Authorization [Member]
Stock Repurchase Program, Authorized Amount | $						$ 75,000
Treasury Stock, Shares, Acquired			322,038
Treasury Stock Acquired, Average Cost Per Share | $ / shares			$ 12.39
Treasury Stock, Value, Acquired, Cost Method | $			$ 4,000
Stock Repurchase Program, Remaining Authorized Repurchase Amount | $	$ 69,000		$ 69,000
November 2004 Authorization [Member]
Stock Repurchase Program, Number of Shares Authorized to be Repurchased								2,000,000
Underwritten Offering [Member]
Stock Issued During Period, Shares, New Issues	17,250,000	13,500,000
Share Price | $ / shares	$ 14.50	$ 13	$ 14.50
Proceeds from Issuance of Common Stock | $	$ 238,900	$ 167,300
Issuance of Stock, Underwriting Discount | $	10,600	7,500
Issuance of Stock, Underwriting Expenses | $	$ 600	$ 900
Common Class A [Member]
Common Stock, Shares Authorized	15,000,000		15,000,000	15,000,000
Common Stock, Voting Rights, Votes Per Share	10		10
Common Stock, Capital Shares Reserved for Future Issuance	1,923,144		1,923,144	257,581
Blank Check Preferred Stock [Member]
Preferred Stock, Shares Authorized	20,000,000		20,000,000