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, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Numerator:
Net loss	$ (10,463)	$ (9,844)	$ (31,063)	$ (26,018)
Less: Net loss attributable to non-controlling interests		(8)		(1,137)
Plus: Cumulative dividends on redeemable convertible preferred shares		(2,052)		(5,313)
Plus: Accretion of bridge units and redeemable convertible preferred shares to redemption value		(188)		(1,133)
Net loss attributable to common stockholders of Spero Therapeutics, Inc.	$ (10,463)	$ (12,076)	$ (31,063)	$ (31,327)
Denominator:
Weighted average common shares outstanding, basic and diluted	17,471,462	335,285	15,417,087	333,402
Net loss per share attributable to common stockholders of Spero Therapeutics, Inc., basic and diluted	$ (0.60)	$ (36.02)	$ (2.01)	$ (93.96)