CONSOLIDATED STATEMENTS OF OPERATIONS AND COMPREHENSIVE LOSS (Unaudited) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Income Statement [Abstract]
Grant revenue	$ 1,153	$ 140
Operating expenses:
Research and development	8,925	5,999
General and administrative	3,044	1,740
Total operating expenses	11,969	7,739
Loss from operations	(10,816)	(7,599)
Other income (expense):
Change in fair value of derivative liabilities		1,199
Interest income and other income (expense), net	172	(11)
Total other income (expense), net	172	1,188
Net loss	(10,644)	(6,411)
Less: Net loss attributable to non-controlling interest		(535)
Net loss attributable to Spero Therapeutics, Inc.	(10,644)	(5,876)
Cumulative dividends on redeemable convertible preferred shares		(1,236)
Accretion of redeemable bridge units and redeemable convertible preferred shares to redemption value		(18)
Net loss attributable to common stockholders of Spero Therapeutics, Inc.	$ (10,644)	$ (7,130)
Net loss per share attributable to common stockholders per share, basic and diluted	$ (0.74)	$ (21.60)
Weighted average common shares outstanding, basic and diluted:	14,369,182	330,075
Comprehensive loss:
Net loss	$ (10,644)	$ (6,411)
Other comprehensive loss:
Unrealized gain (loss) on marketable securities	(29)
Total other comprehensive loss	(29)
Total comprehensive loss	$ (10,673)	$ (6,411)