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Stockholders' Equity (Details) - USD ($) $ / shares in Units, $ in Thousands				3 Months Ended	12 Months Ended
	Jul. 31, 2018	Sep. 22, 2017	Dec. 19, 2016	Mar. 31, 2019	Mar. 05, 2019	Dec. 31, 2018	Dec. 31, 2016	Mar. 31, 2018
Stockholders' Equity
Gross proceeds from December 2016 rights offering							$ 24,822
Public offering \| Ordinary Shares
Stockholders' Equity
Number of shares issued	18,181,818	9,411,765
Shares issued, price per share	$ 2.75	$ 8.50
Gross proceeds from sale of stock	$ 50,000	$ 80,000
Net proceeds from sale of stock	$ 46,300	$ 73,300
At-the-market offering
Stockholders' Equity
Gross proceeds from sale of stock						$ 24,219
At-the-market offering \| Ordinary Shares
Stockholders' Equity
Number of shares issued					7,742,379	4,778,031
Gross proceeds from sale of stock					$ 30,300	$ 24,200
Net proceeds from sale of stock					$ 28,900	$ 23,000
December 2016 Financing \| Common Stock
Stockholders' Equity
Number of shares issued			588,127
Net proceeds from sale of stock			$ 20,600
Gross proceeds from December 2016 rights offering			$ 24,800
ATM Agreement
Stockholders' Equity
Gross proceeds from sale of stock				$ 5,900
ATM Agreement \| Cantor Fitzgerald and Co. \| Ordinary Shares
Stockholders' Equity
Maximum aggregate gross proceeds under the sales agreement								$ 50,000