License Agreements (Details) € in Millions	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2020 USD ($)	Sep. 30, 2019 USD ($)	Sep. 30, 2020 USD ($)	Sep. 30, 2019 USD ($)	Dec. 31, 2019 USD ($)		Sep. 30, 2020 EUR (€)	Nov. 01, 2019 USD ($)
Related Party Transaction [Line Items]
Research and development	$ 51,421,000	$ 39,995,000	$ 140,759,000	$ 95,172,000
Accrued and other current liabilities	39,979,000		39,979,000		$ 23,829,000	[1]
Equity method investment	4,516,000		4,516,000		4,892,000	[1]
Research and development
Related Party Transaction [Line Items]
Collaboration arrangement, expense	1,200,000		2,000,000.0
Notch Therapeutics, Inc.
Related Party Transaction [Line Items]
Equity method investment	$ 4,500,000		$ 4,500,000		4,900,000			$ 5,000,000.0
Ownership percentage	25.00%		25.00%				25.00%	25.00%
Pfizer
Related Party Transaction [Line Items]
Royalty obligation period from date of first sale			12 years
Pfizer | Asset Contribution Agreement
Related Party Transaction [Line Items]
Aggregate potential milestone payments			$ 325,000,000.0
Aggregate potential milestone payments	$ 0	0
Pfizer | Asset Contribution Agreement | Minimum
Related Party Transaction [Line Items]
Aggregate potential milestone payments	30,000,000.0
Pfizer | Asset Contribution Agreement | Maximum
Related Party Transaction [Line Items]
Aggregate potential milestone payments	60,000,000.0
Aggregate potential regulatory and development milestones	840,000,000.0
Cellectis | Research Collaboration and License Agreement
Related Party Transaction [Line Items]
Maximum payments required per product against selected target			185,000,000.0
Cellectis | Pre-Clinical Development Milestone | Research Collaboration and License Agreement
Related Party Transaction [Line Items]
Research and development	0	5,000,000.0	0	5,000,000.0
Cellectis | Development and Sales | Maximum | Research Collaboration and License Agreement
Related Party Transaction [Line Items]
Aggregate potential milestone payments			$ 2,800,000,000
Servier | License and Collaboration Agreement
Related Party Transaction [Line Items]
Research and development		$ 1,500,000		$ 4,500,000
Percentage of development costs payable by the Company			60.00%
Percentage of development cost payable by collaboration partner			40.00%
Recovery of research and development expense	4,100,000		$ 5,900,000
Receivables	3,100,000		3,100,000
Accrued and other current liabilities					2,200,000
Servier | Regulatory Milestone | License and Collaboration Agreement
Related Party Transaction [Line Items]
Aggregate potential milestone receivable	42,000,000.0		42,000,000.0
Servier | Regulatory Milestone | Maximum | License and Collaboration Agreement
Related Party Transaction [Line Items]
Aggregate potential milestone payments			137,500,000
Servier | Sales Milestone | License and Collaboration Agreement
Related Party Transaction [Line Items]
Aggregate potential milestone receivable	82,500,000		82,500,000				€ 70.5
Servier | Sales Milestone | Maximum | License and Collaboration Agreement
Related Party Transaction [Line Items]
Aggregate potential milestone payments			78,000,000.0
Notch Therapeutics, Inc. | Research Collaboration and License Agreement
Related Party Transaction [Line Items]
Aggregate potential milestone payable	$ 7,250,000		7,250,000
Notch Therapeutics, Inc. | License and Collaboration Agreement
Related Party Transaction [Line Items]
Research and development					$ 10,000,000
Collaboration agreement, upfront payment								$ 10,000,000.0
Notch Therapeutics, Inc. | Pre-Clinical Development Milestone | Maximum | License and Collaboration Agreement
Related Party Transaction [Line Items]
Aggregate potential milestone payments			4,000,000.0
Notch Therapeutics, Inc. | Clinical, Regulatory, and Commercial Milestone | Maximum | License and Collaboration Agreement
Related Party Transaction [Line Items]
Aggregate potential milestone payments			$ 283,000,000.0

[1] The balance sheet as of December 31, 2019 is derived from the audited financial statements as of that date.