Restricted Cash Restricted Cash (Tables)

12 Months Ended

Dec. 31, 2017

Restricted Cash and Investments, Noncurrent [Abstract]

Summary of Amounts Designated as Collateral for Letters of Credit

The following table summarizes the amounts designated as collateral for letters of credit related to the lease of office and laboratory space in Cambridge, Massachusetts (collateral amounts are presented in thousands).

Property Location	Approximate Square Footage	Lease Expiration Date	Letter of Credit Amount		Balance Sheet Classification
675 West Kendall Street	78,500	4/30/2018	$	2,412	Current Asset
320 Bent Street	105,000	2/28/2027	748		Non-Current Asset
301 Binney Street, Fifth Floor	80,000	6/29/2025	1,101		Non-Current Asset
301 Binney Street, Fourth Floor	52,000	3/31/2028	1,271		Non-Current Asset
Total			$	5,532