Maximum amounts of investments by parent that would trigger the anti-dilution protection.

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Milestone payments.

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No definition available.

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Termination of agreement notice period.

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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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The equity interest of noncontrolling shareholders, partners or other equity holders in consolidated entity.

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

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The aggregate costs incurred (1) in a planned search or critical investigation aimed at discovery of new knowledge with the hope that such knowledge will be useful in developing a new product or service, a new process or technique, or in bringing about a significant improvement to an existing product or process; or (2) to translate research findings or other knowledge into a plan or design for a new product or process or for a significant improvement to an existing product or process whether intended for sale or the entity's use, during the reporting period charged to research and development projects, including the costs of developing computer software up to the point in time of achieving technological feasibility, and costs allocated in accounting for a business combination to in-process projects deemed to have no alternative future use.

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Equity impact of the value of new stock issued during the period. Includes shares issued in an initial public offering or a secondary public offering.

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Non-Controlling Interests - Spero Europe - Additional Information (Detail) - USD ($)	1 Months Ended	3 Months Ended		9 Months Ended
	Jan. 31, 2016	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017	May 31, 2017
Noncontrolling Interest [Line Items]
Research and development expense		$ 8,459,000	$ 6,910,000	$ 24,758,000	$ 20,366,000
Non-controlling interest		355,000		355,000		$ 355,000
Spero Europe, Ltd. [Member]
Noncontrolling Interest [Line Items]
Termination of agreement notice period	30 days
Non-controlling interest		$ 0		$ 0
Spero Europe, Ltd. [Member] \| Maximum [Member]
Noncontrolling Interest [Line Items]
Non-controlling interest							$ 100,000
Spero Europe, Ltd. [Member] \| Licensing Agreements [Member] \| Anti- dilution Rights [Member]
Noncontrolling Interest [Line Items]
Research and development expense	$ 200,000
Spero Europe, Ltd. [Member] \| Promiliad Biopharma, Inc. [Member]
Noncontrolling Interest [Line Items]
Ownership percentage	5.00%
Common stock issued, aggregate fair value	$ 100,000
Milestone payments	4,100,000
Proceeds from the issuance of common stock, net of issuance costs	0
Maximum amounts of investments for anti-dilution protection	$ 5,000,000
Spero Europe, Ltd. [Member] \| Promiliad Biopharma, Inc. [Member] \| Anti- dilution Rights [Member]
Noncontrolling Interest [Line Items]
Ownership percentage	5.00%
Spero Europe, Ltd. [Member] \| Promiliad Biopharma, Inc. [Member] \| Licensing Agreements [Member]
Noncontrolling Interest [Line Items]
Research and development expense	$ 100,000