Subsequent Events - Additional Information (Details)	1 Months Ended		3 Months Ended									12 Months Ended
	Jan. 31, 2020 USD ($)	May 30, 2018 ft²	Mar. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)	Sep. 30, 2019 USD ($)	Jun. 30, 2019 USD ($)	Mar. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	Sep. 30, 2018 USD ($)	Jun. 30, 2018 USD ($)	Mar. 31, 2018 USD ($)	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2020	Jan. 01, 2019 USD ($)
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Deferred revenue recognized				$ 439,000	$ 344,000	$ 143,000	$ 14,434,000	$ 9,764,000	$ 1,730,000	$ 2,428,000	$ 45,495,000	$ 15,360,000	$ 59,417,000	$ 12,826,000
Type of Revenue [Extensible List]				us-gaap:CollaborativeArrangementMember	us-gaap:CollaborativeArrangementMember	us-gaap:CollaborativeArrangementMember	us-gaap:CollaborativeArrangementMember	us-gaap:CollaborativeArrangementMember	us-gaap:CollaborativeArrangementMember	us-gaap:CollaborativeArrangementMember	us-gaap:CollaborativeArrangementMember	us-gaap:CollaborativeArrangementMember	us-gaap:CollaborativeArrangementMember	us-gaap:CollaborativeArrangementMember
Operating lease, right-of-use asset				$ 14,858,000								$ 14,858,000				$ 16,672,000
Operating lease liabilities				$ 19,930,000								$ 19,930,000
Massachusetts
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Lease term				10 years								10 years
Lease Amendment | Massachusetts
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Lease term, description		In May 2018, the Company executed an amendment to lease an additional 33,526 square feet at 50 Hampshire Street in Cambridge, Massachusetts, with a 10-year term. This additional space became available for occupancy on January 1, 2020
Land subject to additional ground leases | ft²		33,526
Lease term		10 years
Scenario, Forecast | Employee Severance | Operating Expenses
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Severance Costs			$ 1,272,000
Maximum | Massachusetts
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Lease term				10 years								10 years
Minimum | Massachusetts
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Lease term				1 year								1 year
Novartis Collaboration | Novartis Institutes for Biomedical Research, Inc.
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Type of Revenue [Extensible List]												us-gaap:CollaborativeArrangementMember
Novartis Collaboration | Novartis Institutes for Biomedical Research, Inc. | Scenario, Forecast
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Deferred revenue recognized			$ 38,592,000
Sublease agreement with EQRx | Massachusetts
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Term of contract												The term of the sublease agreement commenced in January 2020 and ends on the last day of the 36th calendar month following commencement.
Sublease agreement with EQRx | Massachusetts | Office Space
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Lease commenced date												Jan. 01, 2020
Sublease agreement with EQRx | Lease Amendment | Massachusetts
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Lease term, description		In May 2018, the Company executed an amendment to lease an additional 33,526 square feet at 50 Hampshire Street in Cambridge, Massachusetts, with a 10-year term.
Land subject to additional ground leases | ft²		33,526
Lease term		10 years
Sublease agreement with EQRx | Scenario, Forecast | Massachusetts | Office Space
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Lease term															3 years
Subsequent Event | Employee Severance
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Percentage of reduction of company's workforce	35.00%
Subsequent Event | Sublease agreement with EQRx | Massachusetts
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Operating lease, right-of-use asset	$ 15,003,000
Operating lease liabilities	15,003,000
Milestone Payment | Novartis Collaboration | Novartis Institutes for Biomedical Research, Inc. | Maximum
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Maximum aggregate amount of potential option purchase, option exercise and milestone payments to be currently received				$ 745,000,000								$ 745,000,000
Milestone Payment | Subsequent Event | Novartis Collaboration | Novartis Institutes for Biomedical Research, Inc. | Maximum
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Maximum aggregate amount of potential option purchase, option exercise and milestone payments to be currently received	$ 525,000,000