Income Taxes - Additional Information (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2019 USD ($) Ownership	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)
Income Taxes Disclosure [Line Items]
Increased in valuation allowance	$ 178,900	$ 114,100
Research and development tax credit carryforwards	73,442	39,344
Unrecognized tax benefits	19,487	$ 14,541	$ 5,843
Accrued interest and penalties	300
Research and Development Credits
Income Taxes Disclosure [Line Items]
Unrecognized tax benefits	18,300
Federal
Income Taxes Disclosure [Line Items]
Net operating loss carryforwards	$ 1,005,400
Net operating loss carryforwards, expiration beginning year	2028
Research and development tax credit carryforwards	$ 58,300
Tax credit carryforwards, expiration beginning year	2028
State
Income Taxes Disclosure [Line Items]
Net operating loss carryforwards	$ 851,700
Net operating loss carryforwards, expiration year	2019
Research and development tax credit carryforwards	$ 40,000
Tax credit carryforwards, expiration beginning year	2035
UK
Income Taxes Disclosure [Line Items]
Net operating loss carryforwards	$ 9,400
Research and development tax credit carryforwards	200
Denmark
Income Taxes Disclosure [Line Items]
Net operating loss carryforwards	$ 100
Section 382
Income Taxes Disclosure [Line Items]
Number of ownership changes for income tax purposes | Ownership	4
International
Income Taxes Disclosure [Line Items]
Unrecognized tax benefits	$ 1,200