Note 6 - Income Taxes (Details Textual) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017
Income Tax Disclosure [Abstract]
Provision for income taxes	$ 8,748	$ 9,174	$ 14,534	$ 14,986
Effective Income Tax Rate Reconciliation, Percent	17.60%	26.70%	18.30%	24.50%
Effective income tax rate excluding impact of effect of share based compensation, Percent	20.10%		19.90%
Provisional Toll Charge					$ 2,105