Additional amount of common stock shares agreed to sell upon exercise of option.

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Agreement date.

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- Definition

Cash received as due under collaboration agreement.

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Number of agreements.

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Option payments, and development, regulatory and sales milestone payments.

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- Definition

Trading Period used to calculate share price of common stock surrounding option exercise date.

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No definition available.

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- Definition

Trading period used to calculate weighted average price of common stock listed in stock market.

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- Definition

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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Amount of deferred revenue as of balance sheet date. Deferred revenue represents collections of cash or other assets related to a revenue producing activity for which revenue has not yet been recognized. Generally, an entity records deferred revenue when it receives consideration from a customer before achieving certain criteria that must be met for revenue to be recognized in conformity with GAAP.

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Amount of revenue recognized that was previously reported as deferred or unearned revenue.

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Revenue from multiple-deliverable arrangements that include licensing fees and services revenue. Licensing revenue is consideration received from another party for the right to use, but not own, certain of the entity's intangible assets. Licensing arrangements include, but are not limited to, rights to use a patent, copyright, technology, manufacturing process, software or trademark. Licensing fees are generally, but not always, fixed as to amount and not dependent upon the revenue generated by the licensing party. An entity may receive licensing fees for licenses that also generate royalty payments to the entity. Services revenue may be derived by providing other, nonspecified, services during the reporting period.

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The cash inflow from the additional capital contribution to the entity.

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Number of shares of stock issued as of the balance sheet date, including shares that had been issued and were previously outstanding but which are now held in the treasury.

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Per share or per unit amount of equity securities issued.

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Collaboration and License Agreements - Amgen, Inc - Additional Information (Detail)		1 Months Ended		3 Months Ended			6 Months Ended
	Sep. 28, 2016 USD ($) Agreement $ / shares shares	Nov. 30, 2016 USD ($)	Sep. 30, 2016 USD ($)	Mar. 31, 2018 USD ($) $ / shares	Dec. 31, 2017 USD ($)	Mar. 31, 2017 USD ($)	Mar. 31, 2018 USD ($) $ / shares	Mar. 31, 2017 USD ($)
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Stock issued, price per share \| $ / shares				$ 5.25			$ 5.25
Proceeds from the issuance of common stock							$ 56,585,035	$ 12,691,937
Collaboration and License agreements \| Amgen
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Agreement date	Sep. 28, 2016
Number of agreements \| Agreement	2
ARO-AMG1 Agreement \| Amgen
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Additional amount of common stock shares agreed to sell upon exercise of option	$ 5,000,000
Number of trading days, used to calculate share price of Common Stock, surrounding option exercise date	30 days
Cash received as due under collaboration agreement			$ 5,000,000
Deferred revenue			$ 5,000,000	$ 1,300,000			1,300,000
Deferred revenue amortized into revenue				600,000		$ 600,000	1,300,000	1,300,000
Collaboration and License agreements \| Maximum
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Option payments, and development, regulatory and sales milestone payments.	$ 617,000,000
Collaboration and License agreements \| Amgen
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Cash received as due under collaboration agreement	35,000,000
Proceeds from the issuance of common stock	$ 21,500,000
ARO-LPA (AMG-890) Agreement \| Amgen
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Cash received as due under collaboration agreement		$ 30,000,000
Deferred revenue		$ 30,000,000		0			0
Deferred revenue amortized into revenue					$ 2,700,000	7,800,000		11,500,000
ARO-LPA (AMG-890) and ARO-AMG-1 Agreement \| Amgen
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Revenue generated from work orders				$ 30,000		$ 600,000	$ 200,000	$ 600,000
Common Stock Purchase Agreement \| Amgen
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Agreement date	Sep. 28, 2016
Shares issued \| shares	3,002,793
Stock issued, price per share \| $ / shares	$ 7.16
Common Stock Purchase Agreement \| Amgen \| NASDAQ
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Number of trading days, used to calculate weighted average price of Common Stock, listed in stock market	30 days