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STOCKHOLDERS' EQUITY - Narrative (Detail) $ / shares in Units, $ in Thousands		1 Months Ended		3 Months Ended	9 Months Ended
	Aug. 05, 2020 USD ($)	Apr. 30, 2020 USD ($) $ / shares	Apr. 30, 2019 USD ($) $ / shares shares	Sep. 30, 2019 shares	Sep. 30, 2020 USD ($) shares	Sep. 30, 2019 USD ($) shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Proceeds from exercise of stock options and restricted stock units			$ 136,300		$ 1,187	$ 1,139
Biogen MA, Inc. | Stock Purchase Agreement
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Sale of stock, price per share (in dollars per share) | $ / shares		$ 9.2137
Consideration received on transaction		$ 225,000
Jefferies LLC | At The Market Offering Agreement
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Issuance of common stock under public offering, net of issuance costs (in shares) | shares					0
Sale of stock, percentage of gross proceeds	0.03
Stock offering program, maximum value	$ 150,000
Jefferies LLC | Maximum | At The Market Offering Agreement
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Payments of stock issuance costs	$ 50
Common Stock
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Issuance of common stock under public offering, net of issuance costs (in shares) | shares			12,700,000		24,420,157	12,650,000
Public offering price of common stock issued (in dollars per share) | $ / shares			$ 11.50

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