Quarterly Financial Data (Tables)

12 Months Ended

Dec. 31, 2017

Quarterly Financial Information Disclosure [Abstract]

Quarterly Financial Information

	March 31, 2017			June 30, 2017			September 30, 2017			December 31, 2017
Grant revenue	$	140		$	249		$	597		$	993
Operating expenses		7,739			10,414			10,563			14,993
Net loss and comprehensive loss		(6,411	)		(9,763	)		(9,844	)		(13,868	)
Net loss attributable to Spero Therapeutics, Inc.		(5,876	)		(9,169	)		(9,836	)		(13,862	)
Net loss attributable to common shareholders of Spero Therapeutics, Inc.		(7,130	)		(12,121	)		(12,076	)		(14,770	)
Net loss per share attributable to common shareholders per share, basic and diluted	$	(21.60	)	$	(36.21	)	$	(36.02	)	$	(1.59	)
Weighted average shares outstanding, basic and diluted:		330,075			334,788			335,285			9,273,783

	March 31, 2016			June 30, 2016			September 30, 2016			December 31, 2016
Grant Revenue	$	—		$	—		$	—		$	335
Operating expenses		8,417			8,080			7,914			9,145
Net loss and comprehensive loss		(8,430	)		(8,096	)		(7,918	)		(8,197	)
Net loss attributable to Spero Therapeutics, Inc.		(5,905	)		(6,059	)		(6,316	)		(7,211	)
Net loss attributable to common shareholders of Spero Therapeutics, Inc.		(6,257	)		(7,928	)		(7,410	)		(8,333	)
Net loss per share attributable to common shareholders per share, basic and diluted	$	(21.51	)	$	(25.30	)	$	(23.23	)	$	(25.68	)
Weighted average shares outstanding, basic and diluted:		290,884			313,414			318,948			324,521