Note 12 - Stock Based Compensation - Additional Shares Authorized (Details) - shares	3 Months Ended	12 Months Ended							87 Months Ended
	Mar. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Mar. 31, 2019
The 2010 Stock Option and Incentive Plan [Member]
Total additional shares (in shares)	233,862	106,076	12,623	7,257	5,463	4,128	2,871	2,502	374,782