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Restricted cash
 12 Months Ended
Dec. 31, 2019
Other Assets, Noncurrent [Abstract]  
Restricted cash Restricted cash
As of December 31, 2019, and 2018, the Company maintained letters of credit of $54.5 million and $14.5 million, respectively, which are collateralized with a bank account at a financial institution in accordance with the agreement and consisted of the following (in thousands):
As of December 31,
20192018
60 Binney Street lease$13,763  $13,763  
50 Binney Street sublease40,072  —  
Other leases660  757  
Total restricted cash$54,495  $14,520  
Refer to Note 8, Leases, for further information on the Company's letters of credit.