Amount of cash and cash equivalents restricted as to withdrawal or usage. Cash includes, but is not limited to, currency on hand, demand deposits with banks or financial institutions, and other accounts with general characteristics of demand deposits. Cash equivalents include, but are not limited to, short-term, highly liquid investments that are both readily convertible to known amounts of cash and so near their maturity that they present insignificant risk of changes in value because of changes in interest rates.

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Name:	us-gaap_RestrictedCashAndCashEquivalents
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

Nature of the Business and Basis of Presentation - Liquidity (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Accumulated deficit	$ 335,753	$ 280,403
Cash and Cash Equivalents [Line Items]
Cash, cash equivalents, and restricted cash	91,551	$ 37,682	$ 71,027	$ 59,050
Restricted cash and cash equivalents	1,700
Russian subsidiary \| Unrestricted cash
Cash and Cash Equivalents [Line Items]
Cash, cash equivalents, and restricted cash	$ 400