Exercise price per share or per unit of warrants or rights outstanding.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 235
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.4-08.(i)(4))
-URI http://asc.fasb.org/extlink&oid=26873400&loc=d3e23780-122690

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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.

+ References

No definition available.

+ Details

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Namespace Prefix:	us-gaap_
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- Definition

Number of securities into which the class of warrant or right may be converted. For example, but not limited to, 500,000 warrants may be converted into 1,000,000 shares.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 235
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.4-08.(i)(2))
-URI http://asc.fasb.org/extlink&oid=26873400&loc=d3e23780-122690

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- Definition

Amount of gain (loss) recognized on the income statement for financial instrument classified as a liability measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)(1)
-URI http://asc.fasb.org/extlink&oid=99377789&loc=d3e19207-110258

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Balance Type:	credit
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- Definition

Represents the number of the warrants or rights exercised during the period on net basis.

+ References

No definition available.

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- Definition

Stock Issued During Period Shares Warrants Exercise on Net Basis

+ References

No definition available.

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Namespace Prefix:	xlrn_
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Period Type:

Warrants - Summary of Shares Issuable Upon Exercise of Outstanding Warrants (Details) - USD ($) $ / shares in Units, $ in Thousands	1 Months Ended	12 Months Ended
	Jan. 31, 2018	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Class of Warrant or Right
Number of outstanding warrants (in shares)		39,000	61,000
Weighted-average exercise price per share (in dollars per share)		$ 5.88
Number of warrants exercised (in shares)	21,258
Stock issued during period for warrants exercsied (in shares)	18,449
Increase (decrease) in fair value		$ 52	$ 992	$ (7,300)
Expiring between June 10, 2020 to July 9, 2020
Class of Warrant or Right
Number of outstanding warrants (in shares)		39,000	61,000
Weighted-average exercise price per share (in dollars per share)		$ 5.88