Summary of Significant Accounting Policies - Significant Customers (Details) - Customer Concentration Risk	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Revenue | Strategic Collaborators
Concentration Risk [Line Items]
Concentration risk, percentage	80.00%	91.00%	86.00%
Revenue | Merck
Concentration Risk [Line Items]
Concentration risk, percentage	61.00%	49.00%	31.00%
Revenue | Vertex
Concentration Risk [Line Items]
Concentration risk, percentage	10.00%
Revenue | AstraZeneca
Concentration Risk [Line Items]
Concentration risk, percentage		34.00%	15.00%
Revenue | Alexion
Concentration Risk [Line Items]
Concentration risk, percentage			36.00%
Revenue | Government-Sponsored and Private Organizations
Concentration Risk [Line Items]
Concentration risk, percentage	20.00%	9.00%	14.00%
Revenue | BARDA
Concentration Risk [Line Items]
Concentration risk, percentage	13.00%		10.00%
Accounts Receivable | Merck
Concentration Risk [Line Items]
Concentration risk, percentage	13.00%	30.00%
Accounts Receivable | Vertex
Concentration Risk [Line Items]
Concentration risk, percentage	17.00%	22.00%
Accounts Receivable | BARDA
Concentration Risk [Line Items]
Concentration risk, percentage	54.00%	13.00%
Accounts Receivable | DARPA
Concentration Risk [Line Items]
Concentration risk, percentage		16.00%
Accounts Receivable | Massachusetts Life Sciences Center
Concentration Risk [Line Items]
Concentration risk, percentage		12.00%