Summary of Significant Accounting Policies - Significant Accounting Policies (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended	12 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Jun. 30, 2017	Dec. 31, 2017	Dec. 31, 2016
Research and development incentive income and receivable
Research and development expenses reimbursed (as a percent)	43.50%			43.50%
Research and development incentive income and receivable
Balance at the beginning of the period	$ 1,744	$ 3,053	$ 3,053	$ 3,053
Australian research and development incentive income	646	$ 397		1,715
Cash received for 2016 eligible overseas research and development expenses	(740)			(3,245)
Foreign currency translation	(20)			221
Balance at the end of the period	1,630			1,744	$ 3,053
Overseas research and development incentive income			$ 412		$ 347
Other Assets
Deposits with service providers that are to be applied to future payments	1,076			$ 1,076
Deposit related to operating lease for office space in Cambridge, Massachusetts	$ 35