Income Taxes - Effective Income Tax Rate Reconciliation (Details) - USD ($) $ in Thousands	12 Months Ended
Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Effective Income Tax Rate Reconciliation, Amount [Abstract]
Net loss before tax	$ (213,256)	$ (98,418)	$ (70,430)
Statutory combined U.S. federal and state tax rate (as a percent)	21.00%	21.00%	34.00%
Statutory federal and state taxes	$ (44,784)	$ (20,668)	$ (23,946)
Effect of change in valuation allowance	52,719	25,959	(4,154)
Non-deductible share-based compensation	1,810	884	695
Tax deductions for share-based compensation	(6,917)	(2,924)	(80)
Tax credits	(8,621)	(5,130)	(2,563)
Write off of Methylgene US Inc. net operating loss	0	0	307
Change in tax rate	0	0	303
Tax Cuts and Jobs Act	0	0	28,569
Uncertain tax positions	2,143	1,283	646
Return to provision and other true-ups	(60)	375	394
Non-deductible officers' compensation	3,527	179	0
Other differences	183	42	(171)
Income tax benefit	$ 0	$ 0	$ 0

Effective Income Tax Rate Reconciliation, Combined Federal and State Statutory Income Tax Rate, Percent

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Effective Income Tax Rate Reconciliation, Non-Deductible Officers' Compensation, Amount

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Effective Income Tax Rate Reconciliation, Write Off of Net Operating Loss, Amount

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Amount of income (loss) from continuing operations, including income (loss) from equity method investments, before deduction of income tax expense (benefit), and income (loss) attributable to noncontrolling interest.

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