Commitments and Contingencies - Lease Liabilities (Details) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019
Commitments and Contingencies Disclosure [Abstract]
Current portion included in accrued and other current liabilities	$ 2,873	$ 1,679
Long-term portion of lease liabilities	50,846	51,349	[1]
Total operating lease liabilities	$ 53,719	$ 53,028
Operating Lease, Liability, Current, Statement of Financial Position [Extensible List]	allo:AccruedAndOtherLiabilitiesCurrent	allo:AccruedAndOtherLiabilitiesCurrent

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