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Common stock exchange ratio.

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Net proceeds from issuance public offering.

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Purchase plan offering period.

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Purchase plan purchase period.

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Share based compensation arrangement by share based payment award number of vesting installment.

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Share based compensation arrangement by share based payment award options expiration term.

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Share based compensation arrangement by share based payment award options maximum term for specified stock holder.

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Share based compensation arrangement by share based payment award purchase price of common stock percent for specified stockholder.

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Specified stockholder ownership percentage.

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Stock Offering Program, Maximum Value

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Aggregate number of common shares reserved for future issuance.

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The maximum number of common shares permitted to be issued by an entity's charter and bylaws.

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Number of shares of common stock outstanding. Common stock represent the ownership interest in a corporation.

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Amount of cash outflow to satisfy grantee's tax withholding obligation for award under share-based payment arrangement.

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The maximum number of nonredeemable preferred shares (or preferred stock redeemable solely at the option of the issuer) permitted to be issued by an entity's charter and bylaws.

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Total number of nonredeemable preferred shares (or preferred stock redeemable solely at the option of the issuer) issued to shareholders (includes related preferred shares that were issued, repurchased, and remain in the treasury). May be all or portion of the number of preferred shares authorized. Excludes preferred shares that are classified as debt.

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Aggregate share number for all nonredeemable preferred stock (or preferred stock redeemable solely at the option of the issuer) held by stockholders. Does not include preferred shares that have been repurchased.

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Cash received on stock transaction after deduction of issuance costs.

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Per share amount received by subsidiary or equity investee for each share of common stock issued or sold in the stock transaction.

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Period over which grantee's right to exercise award under share-based payment arrangement is no longer contingent on satisfaction of service or performance condition, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents reported fact of one year, five months, and thirteen days. Includes, but is not limited to, combination of market, performance or service condition.

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The number of grants made during the period on other than stock (or unit) option plans (for example, phantom stock or unit plan, stock or unit appreciation rights plan, performance target plan).

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The weighted average fair value at grant date for nonvested equity-based awards issued during the period on other than stock (or unit) option plans (for example, phantom stock or unit plan, stock or unit appreciation rights plan, performance target plan).

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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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Number of shares authorized for issuance under share-based payment arrangement.

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Amount of accumulated difference between fair value of underlying shares on dates of exercise and exercise price on options exercised (or share units converted) into shares.

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Gross number of share options (or share units) granted during the period.

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Amount by which current fair value of underlying stock exceeds exercise price of fully vested and expected to vest exercisable or convertible options. Includes, but is not limited to, unvested options for which requisite service period has not been rendered but that are expected to vest based on achievement of performance condition, if forfeitures are recognized when they occur.

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Percentage of vesting of award under share-based payment arrangement.

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Intrinsic value of vested award under share-based payment arrangement. Excludes share and unit options.

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Amount of difference between fair value of the underlying shares reserved for issuance and exercise price of vested portions of options outstanding and currently exercisable.

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Weighted average remaining contractual term for vested portions of options outstanding and currently exercisable or convertible, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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Purchase price of common stock expressed as a percentage of its fair value.

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Per share or per unit amount of equity securities issued.

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Number of shares used to settle grantee's tax withholding obligation for award under share-based payment arrangement.

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Number of new stock issued during the period.

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Ratio applied to the conversion of stock split, for example but not limited to, one share converted to two or two shares converted to one.

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Stockholders' Equity - Additional Information (Detail) $ / shares in Units, $ in Thousands		1 Months Ended					12 Months Ended
	Aug. 05, 2020 USD ($)	Apr. 30, 2020 USD ($) $ / shares	Apr. 30, 2020 USD ($) $ / shares	Apr. 30, 2019 USD ($) $ / shares shares	Jun. 30, 2018 shares	Apr. 30, 2018 USD ($) $ / shares shares	Dec. 31, 2020 USD ($) installment $ / shares shares	Dec. 31, 2019 USD ($) $ / shares shares	Dec. 31, 2018 USD ($) $ / shares shares	Jun. 30, 2020 shares	Jun. 29, 2020 shares	May 31, 2020 shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Preferred shares authorized (in shares)							5,000,000	5,000,000
Preferred stock issued (in shares)							0	0
Preferred stock outstanding (in shares)							0	0
Common stock, shares authorized (in shares)							320,000,000	320,000,000		320,000,000	160,000,000
Common stock, shares outstanding (in shares)							142,063,203	115,972,708
Net proceeds from issuance of common stock, after deducting offering expenses \| $							$ 0	$ 136,308	$ 215,758
Additional equity award (in shares)							4,563,425
Stock option maximum term							6 years 5 months 23 days
Specified stockholder ownership percentage							10.00%
Purchase price of common stock percent under stock incentive plans for specified stockholder							110.00%
Stock option maximum term for specified stockholder							5 years
Share of common stock subject to a stock option or stock appreciation right (in shares)							100.00%
Intrinsic value of options exercised \| $							$ 5,400	4,700	27,000
Options vested and expected to vest, aggregate intrinsic value \| $							69,600	7,500	24,500
Options exercisable, aggregate intrinsic value \| $							$ 29,025
Restricted stock units awarded (in shares)							2,517,101
Total payments for the employees' tax obligations to taxing authorities \| $							$ 765	$ 422	$ 254
Shares reserved for issuance of future awards (in shares)							2,483,218
Share-Based Compensation Awards, Tranche 2
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock options granted vesting percentage							33.33%
Share-Based Compensation Awards, Tranche 3
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock options granted vesting percentage							33.33%
2018 Stock Incentive Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Additional equity award (in shares)						0
Percent of fair value per share of common stock on option grant date							100.00%
Stock options vesting period							4 years
Stock options expiration term							10 years
Fixed ratio shares of common stock							1.33
Shares reserved for issuance of future awards (in shares)							10,942,576
2010 Employee Stock Purchase Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Percent of fair value per share of common stock on option grant date					85.00%
Stock reserved for issuance under plan (in shares)					4,600,000
Purchase plan offering period					2 years
Purchase plan purchase period					6 months
Restated 2018 Stock Incentive Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Increase in capital shares reserved for future issuance (in shares)												9,900,000
Maximum \| 2018 Stock Incentive Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock option maximum term							10 years
Employee Stock Options \| 2018 Stock Incentive Plan \| Share-Based Compensation Awards, Tranche 1
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock options granted vesting percentage							25.00%
Employee Stock Options \| 2018 Stock Incentive Plan \| Share-Based Compensation Awards, Tranche 2
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock options granted vesting percentage							2.08%
Restricted Stock Units (RSUs)
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Restricted stock units awarded (in shares)							2,517,101	834,745	346,055
Grant date fair value per award of restricted stock units (in dollars per share) \| $ / shares							$ 8.06	$ 9.49	$ 17.87
Share-based compensation arrangement by share-based payment award, number of vesting installment \| installment							3
Aggregate fair value of RSUs vested in period \| $							$ 3,700	$ 2,000	$ 600
Shares withheld from issuance in order to pay employee taxes (in shares)							90,617	39,160	20,193
Restricted Stock Units (RSUs) \| Share-Based Compensation Awards, Tranche 1
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock options granted vesting percentage							33.33%
Jefferies LLC \| ATM Agreement
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Issuance of common stock, net of issuance costs (in shares)							0
Stock offering program, maximum value \| $	$ 150,000
Stock Purchase Agreement \| Biogen MA Inc
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Sale of stock, price per share (in dollars per share) \| $ / shares		$ 9.2137	$ 9.2137
Consideration received from sale of stock \| $		$ 225,000	$ 225,000
Common Stock
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Issuance of common stock, net of issuance costs (in shares)				12,700,000		14,200,000	24,420,157	12,650,000	14,156,500
Public offering price of common stock issued (in dollars per share) \| $ / shares				$ 11.50		$ 16.25
Net proceeds from issuance of common stock, after deducting offering expenses \| $				$ 136,300		$ 215,800