Income Taxes - Narrative (Details) - USD ($)	3 Months Ended	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2019	Dec. 31, 2017	Dec. 31, 2016
Tax Carryforwards [Line Items]
Decrease in valuation allowance		$ 30,500,000
Income tax (benefit) resulting in the release of valuation allowance	$ (1,560,000,000)
Valuation allowance	168,491,000	205,192,000
Unrecognized tax benefits	$ 19,549,000	33,920,000	$ 3,814,000	$ 0
Income tax penalties and interest accrued		0
Domestic and Foreign Tax Authority
Tax Carryforwards [Line Items]
Valuation allowance		205,200,000
Domestic Tax Authority
Tax Carryforwards [Line Items]
Operating loss carryforwards		1,500,000,000
Operating loss carryforwards, subject to expiration		1,300,000,000
Operating loss carryforwards, not subject to expiration		181,100,000
Tax credit carryforwards		399,000,000.0
State and Local Jurisdiction
Tax Carryforwards [Line Items]
Operating loss carryforwards		903,700,000
Tax credit carryforwards		161,700,000
Foreign Tax Authority
Tax Carryforwards [Line Items]
Operating loss carryforwards		896,900,000
Operating loss carryforwards, subject to expiration		16,000,000.0
Operating loss carryforwards, not subject to expiration		880,900,000
Deferred Tax Assets
Tax Carryforwards [Line Items]
Unrecognized tax benefits		13,400,000
Accounts Payable and Accrued Liabilities
Tax Carryforwards [Line Items]
Unrecognized tax benefits		20,500,000
Semma Therapeutics, Inc.
Tax Carryforwards [Line Items]
Business combination, recognized identifiable assets acquired and liabilities assumed, deferred tax liabilities, current		$ 54,200,000