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Commitments and Contingencies - Additional Information (Detail)					1 Months Ended	3 Months Ended
	Jan. 01, 2017 ft²	Oct. 01, 2015	Apr. 15, 2015 Option	Jul. 28, 2014	Jan. 31, 2017	Mar. 31, 2018 USD ($)	Mar. 31, 2017 USD ($)
Other Commitments [Line Items]
Operating lease, rental expense						$ 100,000	$ 100,000
Subsequent Licensed Product [Member]
Other Commitments [Line Items]
Licensing fees per product maximum						1,300,000
Development Based Milestones [Member]
Other Commitments [Line Items]
Milestone payments due						$ 0	$ 0
Boston Office Space [Member]
Other Commitments [Line Items]
Lease expiration date				Jul. 31, 2017	Jul. 31, 2020	Jul. 31, 2020
Lease extension period				3 years
Boston Office Space [Member] | Unrelated Third Party Member
Other Commitments [Line Items]
Subleased area | ft²	2,976
Sublease expiration date	Dec. 31, 2018
Sublease rental income						$ 200,000
Additional Boston Office Space [Member]
Other Commitments [Line Items]
Lease expiration date			Jul. 31, 2017			Jul. 31, 2020
Lease extension period			3 years
Number of options to extend lease | Option			2
San Diego Office Space [Member]
Other Commitments [Line Items]
Lease expiration date		Sep. 30, 2019
Lease extension period		5 years
Maximum [Member] | Pre Commercialization Milestones [Member]
Other Commitments [Line Items]
Milestone payments						$ 12,300,000
Maximum [Member] | Product Commercialization Milestones [Member]
Other Commitments [Line Items]
Milestone payments						$ 12,500,000

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- Definition

Date which lease or group of leases is set to expire, in CCYY-MM-DD format.

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- Definition

Term of lessee's operating lease renewal, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents reported fact of one year, five months, and thirteen days.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 842
-SubTopic 20
-Section 50
-Paragraph 3
-Subparagraph (a)(3)
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- Definition

Rental expense for the reporting period incurred under operating leases, including minimum and any contingent rent expense, net of related sublease income.

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Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 840
-SubTopic 20
-Section 50
-Paragraph 1
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- Definition

The total amount of sublease rental income recognized during the period that reduces the entity's rent expense incurred under operating leases.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
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-Section 55
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Maximum fee the Company is obligated to pay per licensed product.

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Amount of payment due for milestones that have been achieved

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Amount of payment due upon reaching milestone.

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The number of options available to extend an operating lease.

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