Note 7 - Fair Value Measurements (Details Textual) - USD ($)		3 Months Ended	12 Months Ended
	Jan. 01, 2019	Mar. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Current Period Reclassification Adjustment	$ 865,000	$ 865,000
Additional Paid-in Capital [Member]
Current Period Reclassification Adjustment		2,896,000
Reclassification of Warrant Liability to Additional Paid-in Capital [Member]
Current Period Reclassification Adjustment		$ 865,000
Gains from Revaluating Warrants			$ 1,500,000	$ 500,000
Reclassification of Warrant Liability to Additional Paid-in Capital [Member] | Additional Paid-in Capital [Member]
Cumulative Effect of New Accounting Principle in Period of Adoption	2,000,000
Reclassification of Warrant Liability to Additional Paid-in Capital [Member] | Retained Earnings [Member]
Cumulative Effect of New Accounting Principle in Period of Adoption	$ 2,000,000