Note 8 - Income Taxes (Details Textual) - USD ($)		3 Months Ended		6 Months Ended		12 Months Ended
	Dec. 22, 2017	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2018	Dec. 31, 2017
Unrecognized Tax Benefits that Would Impact Effective Tax Rate		$ 2,600,000		$ 2,600,000			$ 2,600,000
Unrecognized Tax Benefits, Income Tax Penalties Accrued		0		0
Unrecognized Tax Benefits, Interest on Income Taxes Accrued		166,000		166,000			166,000
Effective Income Tax Rate Reconciliation, at Federal Statutory Income Tax Rate, Percent	35.00%
Income Tax Expense (Benefit), Continuing Operations, Adjustment of Deferred Tax (Asset) Liability							$ (82,900,000)
Effective Income Tax Rate Reconciliation, Share-based Compensation, Excess Tax Benefit, Amount		$ 200,000	$ 900,000	$ 200,000	$ 2,000,000
Domestic Tax Authority [Member] | Internal Revenue Service (IRS) [Member]
Income Tax Examination, Year under Examination				2015
Domestic Tax Authority [Member] | Latest Tax Year [Member]
Open Tax Year				2017
Domestic Tax Authority [Member] | Earliest Tax Year [Member]
Open Tax Year				2014
State and Local Jurisdiction [Member] | Latest Tax Year [Member]
Open Tax Year				2017
State and Local Jurisdiction [Member] | Earliest Tax Year [Member]
Open Tax Year				2013
Scenario, Forecast [Member]
Effective Income Tax Rate Reconciliation, at Federal Statutory Income Tax Rate, Percent						21.00%