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Quarterly Financial Data (Detail) - USD ($) $ / shares in Units, $ in Thousands | 3 Months Ended | 12 Months Ended
--- | --- | ---
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
Quarterly Financial Information Disclosure [Abstract] |  |  |  |  |  |  |  |  |  |  |  |
Grant revenue | $ 1,153 | $ 993 | $ 597 | $ 249 | $ 140 | $ 335 |  |  |  | $ 1,979 | $ 335 |
Operating expenses | 11,969 | 14,993 | 10,563 | 10,414 | 7,739 | 9,145 | $ 7,914 | $ 8,080 | $ 8,417 | 43,709 | 33,556 | $ 13,327
Net loss and comprehensive 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)
Net loss attributable to Spero Therapeutics, Inc. | (10,644) | (13,862) | (9,836) | (9,169) | (5,876) | (7,211) | (6,316) | (6,059) | (5,905) | (38,743) | (25,491) | (10,154)
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)
Net loss per share attributable to common shareholders per share, 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)
Weighted average 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

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