Revenue from Contracts with Customers - Schedule of Impact of Changes in Accounting Standard (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended	6 Months Ended
Jun. 30, 2019	Mar. 31, 2019	Jun. 30, 2018	Mar. 31, 2018	Jun. 30, 2019	Jun. 30, 2018	Dec. 31, 2018
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Revenue	$ 30,345		$ 24,999		$ 58,033	$ 48,084
Total revenue	30,345		24,999		58,033	48,084
Net loss	$ (20,037)	$ (21,898)	$ (20,601)	$ (19,202)	$ (41,935)	$ (39,803)
Net loss per share - basic and diluted (in dollars per share)	$ (0.57)		$ (0.80)		$ (1.26)	$ (1.55)
Deferred revenue, current portion	$ 6,033				$ 6,033		$ 9,890
Accumulated deficit	$ (433,191)				$ (433,191)		$ (391,256)

Amount of deferred income and obligation to transfer product and service to customer for which consideration has been received or is receivable, classified as current.

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Amount of revenue recognized from goods sold, services rendered, insurance premiums, or other activities that constitute an earning process. Includes, but is not limited to, investment and interest income before deduction of interest expense when recognized as a component of revenue, and sales and trading gain (loss).

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