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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-Publisher FASB
-Name Accounting Standards Codification
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Name:	us-gaap_RestrictedCashAndCashEquivalents
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

Restricted Cash (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017	Jun. 30, 2017
Restricted Cash
Restricted Cash and Cash Equivalents	$ 4,557		$ 4,767
Restricted cash included in long-term assets	4,557	$ 4,555
Cambridge Massachusetts, 38 Sidney Street and 45 Sidney Street Leases
Restricted Cash
Restricted cash included in long-term assets	4,600	4,600
Restricted by bank
Restricted Cash
Restricted Cash and Cash Equivalents	$ 4,600	$ 4,800