MAJOR CUSTOMERS, PARTNERSHIPS AND STRATEGIC ALLIANCES - Biogen MA, Inc. (Detail) - Biogen MA, Inc.	1 Months Ended			3 Months Ended			9 Months Ended		12 Months Ended
	May 31, 2020 USD ($)	Apr. 30, 2020 USD ($) $ / shares shares	Feb. 29, 2020 USD ($) product_target	Sep. 30, 2021 USD ($)	Mar. 31, 2021 USD ($)	Sep. 30, 2020 USD ($)	Sep. 30, 2021 USD ($) product_target	Sep. 30, 2020 USD ($)	Dec. 31, 2020 USD ($)
Stock purchase agreement
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Number of shares issued in transaction (in shares) | shares		24,420,157
Sale of stock, price per share (in dollars per share) | $ / shares		$ 9.2137
Consideration received on transaction									$ 225,000,000
Standstill restriction period			3 years
Standstill restriction, ownership threshold percentage ownership percentage			5.00%
Agreement restriction, percentage of shares held			50.00%
Voting provisions expiration period			2 years
Voting provisions, ownership threshold percentage			5.00%
Collaborative arrangement transaction price		$ 204,600,000
Excess consideration received on transaction					$ 79,600,000
Collaboration agreement, equity issued									145,400,000
Deferred revenue				$ 161,300,000			$ 161,300,000		183,200,000
Recognized portion of equity issued									2,900,000
Collaboration And License Agreement
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Consideration received on transaction									204,600,000
Proceeds from collaborators	$ 125,000,000								125,000,000
Maximum milestone payment receivable			$ 2,370,000,000
Number of product targets | product_target			12
Number of additional product targets | product_target			9
Target selection period			5 years
Number of product targets selected | product_target			3
Research period			7 years
Maximum number of product targets replaced | product_target							10
Financial advisory fees									$ 7,000,000
Percent of initial recognition									2.00%
Portion of contract asset recognized									$ 4,100,000
Amortization				$ 100,000		$ 100,000	$ 400,000	$ 300,000
Collaboration And License Agreement | Pre-approval milestone
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Maximum milestone payment receivable			$ 925,000,000
Collaboration And License Agreement | Sales-based milestone
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Maximum milestone payment receivable			$ 1,450,000,000