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Net Loss per Share (Tables) | 12 Months Ended
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Dec. 31, 2017
Earnings Per Share [Abstract] |
Summary of Basic and Diluted Net Loss per Share Attributable to Common Stockholders | Basic and diluted net loss per share attributable to common stockholders of Spero Therapeutics, Inc. was calculated as follows (in thousands, except share and per share amounts):
|  | Year Ended December 31, |  |
|  | 2017 |  |  | 2016 |  |  | 2015 |  |
Numerator: |  |  |  |  |  |  |  |  |  |  |  |  |
Net loss |  | $ | (39,886 | ) |  | $ | (32,641 | ) |  | $ | (13,153 | ) |
Less: Net loss attributable to non-controlling interests |  |  | (1,143 | ) |  |  | (7,150 | ) |  |  | (2,999 | ) |
Plus: Cumulative dividends on redeemable convertible preferred shares |  |  | (6,146 | ) |  |  | (3,441 | ) |  |  | (932 | ) |
Plus: Accretion of bridge units and redeemable convertible preferred shares to redemption value |  |  | (1,208 | ) |  |  | (996 | ) |  |  | (2,341 | ) |
Net loss attributable to common stockholders of Spero Therapeutics, Inc. |  | $ | (46,097 | ) |  | $ | (29,928 | ) |  | $ | (13,427 | ) |
Denominator: |  |  |  |  |  |  |  |  |  |  |  |  |
Weighted average common shares outstanding, basic and diluted |  |  | 2,586,865 |  |  |  | 312,169 |  |  |  | 252,807 |  |
Net loss per share attributable to common stockholders of Spero Therapeutics, Inc., basic and diluted |  | $ | (17.82 | ) |  | $ | (95.87 | ) |  | $ | (53.11 | ) |
Summary of Shares Excluded From the Computation of Diluted Net Loss Per Share Attributable to Common Stockholders | The Company excluded the following potential common shares, presented based on amounts outstanding at each period end, from the computation of diluted net loss per share attributable to common stockholders for the periods indicated because including them would have had an anti-dilutive effect:
|  | December 31, |  |
|  | 2017 |  |  | 2016 |  |  | 2015 |  |
Options to purchase common stock |  |  | 2,011,296 |  |  |  | — |  |  |  | — |  |
Redeemable convertible preferred shares (as converted to common shares) |  |  | — |  |  |  | 2,229,518 |  |  |  | 1,257,213 |  |
Incentive units |  |  | — |  |  |  | 413,266 |  |  |  | 171,758 |  |
|  |  | 2,011,296 |  |  |  | 2,642,784 |  |  |  | 1,428,971 |  |

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- Definition Tabular disclosure of securities (including those issuable pursuant to contingent stock agreements) that could potentially dilute basic earnings per share (EPS) in the future that were not included in the computation of diluted EPS because to do so would increase EPS amounts or decrease loss per share amounts for the period presented, by antidilutive securities. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 260 -SubTopic 10 -Section 50 -Paragraph 1 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=6371337&loc=d3e3550-109257 + Details
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- Definition Tabular disclosure of an entity's basic and diluted earnings per share calculations, including a reconciliation of numerators and denominators of the basic and diluted per-share computations for income from continuing operations. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 260 -SubTopic 10 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6371337&loc=d3e3550-109257 + Details
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