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Net Loss per Share - Summary of Basic and Diluted Net Loss per Share Attributable to Common Stockholders (Detail) - USD ($) $ / shares in Units, $ in Thousands | 3 Months Ended | 12 Months Ended
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Mar. 31, 2018 | Dec. 31, 2017 | Sep. 30, 2017 | Jun. 30, 2017 | Mar. 31, 2017 | Dec. 31, 2016 | Sep. 30, 2016 | Jun. 30, 2016 | Mar. 31, 2016 | Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015
Numerator: |  |  |  |  |  |  |  |  |  |  |  |
Net loss | $ (10,644) | $ (13,868) | $ (9,844) | $ (9,763) | $ (6,411) | $ (8,197) | $ (7,918) | $ (8,096) | $ (8,430) | $ (39,886) | $ (32,641) | $ (13,153)
Less: Net loss attributable to non-controlling interests |  |  |  |  | (535) |  |  |  |  | (1,143) | (7,150) | (2,999)
Plus: Cumulative dividends on redeemable convertible preferred shares |  |  |  |  | (1,236) |  |  |  |  | (6,146) | (3,441) | (932)
Plus: Accretion of bridge units and redeemable convertible preferred shares to redemption value |  |  |  |  | (18) |  |  |  |  | (1,208) | (996) | (2,341)
Net loss attributable to common shareholders of Spero Therapeutics, Inc. | $ (10,644) | $ (14,770) | $ (12,076) | $ (12,121) | $ (7,130) | $ (8,333) | $ (7,410) | $ (7,928) | $ (6,257) | $ (46,097) | $ (29,928) | $ (13,427)
Denominator: |  |  |  |  |  |  |  |  |  |  |  |
Weighted average common shares outstanding, basic and diluted | 14,369,182 | 9,273,783 | 335,285 | 334,788 | 330,075 | 324,521 | 318,948 | 313,414 | 290,884 | 2,586,865 | 312,169 | 252,807
Net loss per share attributable to common stockholders of Spero Therapeutics, Inc., basic and diluted | $ (0.74) | $ (1.59) | $ (36.02) | $ (36.21) | $ (21.60) | $ (25.68) | $ (23.23) | $ (25.30) | $ (21.51) | $ (17.82) | $ (95.87) | $ (53.11)

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- Definition Accretion of redeemable bridge units and redeemable convertible preferred shares to redemption value. + References No definition available. + Details
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- Definition The amount of net income or loss for the period per each share in instances when basic and diluted earnings per share are the same amount and reported as a single line item on the face of the financial statements. Basic earnings per share is the amount of net income or loss for the period per each share of common stock or unit outstanding during the reporting period. Diluted earnings per share includes the amount of net income or loss for the period available to each share of common stock or common unit outstanding during the reporting period and to each share or unit that would have been outstanding assuming the issuance of common shares or units for all dilutive potential common shares or units outstanding during the reporting period. + References No definition available. + Details
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- Definition Amount of Net Income (Loss) attributable to noncontrolling interest. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 810 -SubTopic 10 -Section 55 -Paragraph 4J -URI http://asc.fasb.org/extlink&oid=84234705&loc=SL4591551-111686 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 810 -SubTopic 10 -Section 50 -Paragraph 1A -Subparagraph (a)(2) -URI http://asc.fasb.org/extlink&oid=84161450&loc=SL4573702-111684 + Details
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