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		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Numerator:
Net loss	$ (9,844)	$ (7,918)	$ (26,018)	$ (24,444)
Less: Net loss attributable to non-controlling interests	(8)	(1,602)	(1,137)	(6,164)
Plus: Cumulative dividends on redeemable convertible preferred shares	(2,052)	(916)	(5,313)	(2,523)
Plus: Accretion of redeemable bridge units and redeemable convertible preferred stock to redemption value	(188)	(178)	(1,133)	(792)
Net loss attributable to common stockholders of Spero Therapeutics, Inc.	$ (12,076)	$ (7,410)	$ (31,327)	$ (21,595)
Denominator:
Weighted average common shares outstanding-basic and diluted	335,285	318,948	333,402	307,852
Net loss per share attributable to common stockholders of Spero Therapeutics, Inc.-basic and diluted	$ (36.02)	$ (23.23)	$ (93.96)	$ (70.15)