Restructuring Costs - Accrual Rollforward (Details) $ in Thousands	3 Months Ended	6 Months Ended	12 Months Ended
Jun. 30, 2019 USD ($)	Jun. 30, 2019 USD ($)	Jun. 30, 2019 USD ($)
Restructuring costs
Charges incurred	$ 45	$ 176
Cambridge, Massachusetts facility operations
Restructuring costs
Accrued restructuring balance, beginning of period		1,147
Charges incurred		173	$ 3,300
Cash payments		(859)
Accrued restructuring balance, end of period	461	461	461
Restructuring Reserve, Noncurrent	100	100	100
Cambridge, Massachusetts facility operations \| Employee Severance and Benefits
Restructuring costs
Accrued restructuring balance, beginning of period		1,147
Charges incurred		173
Cash payments		(859)
Accrued restructuring balance, end of period	$ 461	$ 461	$ 461

Amount of cash payments made as the result of exit or disposal activities. Excludes payments associated with a discontinued operation or an asset retirement obligation.

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Amount of expenses associated with exit or disposal activities pursuant to an authorized plan. Excludes expenses related to a discontinued operation or an asset retirement obligation.

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Carrying amount (including both current and noncurrent portions of the accrual) as of the balance sheet date pertaining to a specified type of cost associated with exit from or disposal of business activities or restructuring pursuant to a duly authorized plan.

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Carrying amount as of the balance sheet date of known and estimated costs associated with exit from or disposal of business activities or restructurings pursuant to a duly authorized plan, which are expected to be paid after one year or beyond the next operating cycle, if longer. Costs of such activities include those for one-time termination benefits, termination of an operating lease or other contract, consolidating or closing facilities, and relocating employees, and costs associated with an ongoing benefit arrangement, but excludes costs associated with the retirement of a long-lived asset.

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A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period.

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