Note 2 - Basis of Presentation (Details Textual) - USD ($)	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017	Dec. 31, 2016
Cash and Cash Equivalents, Period Increase (Decrease), Total	$ 183,924,000	$ (364,504,000)
Cash and Cash Equivalents, at Carrying Value, Ending Balance	$ 723,725,000	820,580,000	$ 539,801,000	$ 1,185,084,000
Previously Reported [Member]
Net Cash Provided by (Used in) Investing Activities, Continuing Operations, Total		251.60
Cash and Cash Equivalents, Period Increase (Decrease), Total		(27,500,000)
Cash and Cash Equivalents, at Carrying Value, Ending Balance		$ 190,500,000		$ 218,000,000