LICENSE AGREEMENTS AND COMMITMENTS - SADA License Agreement (Details) $ in Thousands		1 Months Ended				3 Months Ended			9 Months Ended		12 Months Ended
	Apr. 15, 2020 USD ($) individual	Aug. 31, 2020 shares	Jul. 31, 2020 individual	Apr. 30, 2020 USD ($) individual shares	Aug. 31, 2015 individual	Sep. 30, 2020 USD ($) shares	Jun. 30, 2020 USD ($) individual shares	Sep. 30, 2019 USD ($) shares	Sep. 30, 2020 USD ($)	Sep. 30, 2019 USD ($)	Dec. 31, 2019 USD ($) individual shares	Dec. 31, 2018 individual shares	Dec. 31, 2017 shares	Dec. 31, 2016 shares	Dec. 31, 2015 shares
Agreements
Non-cash expense in connection with equity issuance to inventors									$ 7,376
Financial statement information
Expense						$ 32,641		$ 24,359	99,841	$ 59,246
Accrued liabilities, current						7,570			7,570		$ 4,550
Accrued liabilities, noncurrent						$ 2,466			2,466		$ 1,921
Common Stock
Agreements
Issuance of common stock to non-employees (in shares) | shares						400,000	256,896	400,000
Nonemployees | Researchers, Employees of MSK
Agreements
Number of individuals | individual					2						1	2
Nonemployees | Researchers, Employees of MSK | Common Stock
Agreements
Issuance of common stock to non-employees (in shares) | shares		400,000									400,000	544,000	448,000	448,000	560,000
SADA License Agreement
Agreements
Number of employees acquired | individual	0
Upfront payment	$ 1,995
Stock grant expense							$ 3,331
Issuance of common stock to strategic partner							$ 1,331
Issuance of common stock to strategic partner (in shares) | shares							42,900
Accrued stock issuance costs							$ 2,000
Annual minimum royalties payable (non-refundable)	40
Annual minimum royalties payable from tenth anniversary (non-refundable)	60
Maximum sales based milestone payments due for each construct previously generated	$ 60,000
Milestone expenses						$ 0			605
Milestone period	36 months
Financial statement information
Cash paid									1,995
Expense									13,307
Accrued liabilities, current						1,000			1,000
Accrued liabilities, noncurrent						1,605			1,605
Milestones
Maximum Clinical Milestones	$ 4,730					4,730			4,730
Maximum Regulatory Milestones	18,125					18,125			18,125
Maximum Sales-based Milestones	$ 23,750					$ 23,750			$ 23,750
SADA License Agreement | Researchers, Employees of MSK
Agreements
Number of individuals | individual			2
SADA License Agreement | Nonemployees | Researchers, Employees of MSK
Agreements
Non-cash expense in connection with equity issuance to inventors				$ 7,376			$ 7,376
Number of individuals | individual				2			2
SADA License Agreement | Nonemployees | Researchers, Employees of MSK | Common Stock
Agreements
Issuance of common stock to non-employees (in shares) | shares				213,996			213,996