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Note 11 - Capital Stock (Details Textual) - USD ($)			1 Months Ended	2 Months Ended	8 Months Ended	9 Months Ended		13 Months Ended	18 Months Ended
	May 15, 2018	Mar. 06, 2018	Sep. 30, 2019	May 15, 2018	Aug. 20, 2019	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Aug. 20, 2019	Aug. 13, 2019	Dec. 31, 2018	Aug. 03, 2018	May 04, 2018	Feb. 28, 2018	Feb. 22, 2018
Proceeds from Issuance of Common Stock						$ 29,229,522	$ 26,696,400
Payments of Stock Issuance Costs						$ 924,742	$ 1,555,300
Common Stock, Shares Authorized													40,000,000
Preferred Stock, Shares Authorized			1,000,000			1,000,000		1,000,000			1,000,000		1,000,000
Class of Warrant or Right, Outstanding			105,556			105,556		105,556			105,556
Common Class A [Member]
Sale of Stock, Value, Available to Issue															$ 150,000,000
Sale of Stock, Price Per Share															$ 23.30
Share Price															$ 23.29
Stock Issued During Period, Shares, New Issues	3,137,768	3,300,000
Proceeds from Issuance of Common Stock	$ 73,110,000	$ 76,890,000		$ 150,000,000
Common Stock, Shares Authorized			38,838,884			38,838,884		38,838,884			38,838,884		38,838,884
Class of Warrant or Right, Outstanding			784			784		784
Common Class A [Member] | Shelf Registration [Member]
Sale of Stock, Shelf Registration, Value, Available to Issue														$ 200,000,000
Sale of Stock, Shelf Registration, Value, Incremental Amount Available to Issue														$ 50,000,000
Common Class A [Member] | Shelf Registration [Member] | Cowen and Company LLC [Member]
Stock Issued During Period, Shares, New Issues			310,152		942,223				2,141,452
Proceeds from Issuance of Common Stock					$ 22,753,943			$ 6,475,579	$ 49,999,625
Sale of Stock, Shelf Registration, Value, Available to Issue										$ 75,000,000
Compensation, Percentage of Common Stock Sales										3.00%				3.00%
Proceeds from Issuance of Common Stock, Net					22,059,015			$ 6,245,765
Payments of Stock Issuance Costs			$ 229,814		$ 694,928
Common Class B [Member]
Common Stock, Shares Authorized			1,161,116			1,161,116		1,161,116			1,161,116		1,161,116
Class of Warrant or Right, Outstanding			105,556			105,556		105,556
Class of Warrant or Right, Number of Securities Called by Warrants or Rights												784

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