Income Taxes - Deferred Assets and Liabilities, CFDs (Details) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Components of net deferred tax assets
Net operating loss carryforwards	$ 16,836	$ 5,750
Capitalized start-up costs	8,840	4,401
Research and development tax credit carryforwards	1,480	500
Capitalized research and development expenses	594	611
Intangible assets	1,403	1,650
Stock‑based compensation expenses	2,629	373
Property and equipment	199
Other	2	1
Total deferred tax assets	31,983	13,286
Section 481 (a) adjustment	(1,257)
Total deferred tax liabilities	(1,257)
Valuation allowance	(30,726)	(13,286)	$ (6,444)	$ (2,899)
United Kingdom Tax Authority
Components of net deferred tax assets
Operating Loss Carryforwards	7,972
Federal
Components of net deferred tax assets
Research and development tax credit carryforwards		$ 1,488
Operating Loss Carryforwards	38,137
Federal | Research Tax Credit Carryforward [Member]
Components of net deferred tax assets
Tax Credit Carryforward, Amount	1,488
State
Components of net deferred tax assets
Operating Loss Carryforwards	38,137
State | Research Tax Credit Carryforward [Member]
Components of net deferred tax assets
Tax Credit Carryforward, Amount	$ 1,488