CONSOLIDATED STATEMENTS OF OPERATIONS AND COMPREHENSIVE LOSS (Unaudited) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Income Statement [Abstract]
Grant revenue	$ 658	$ 597	$ 2,274	$ 986
Operating expenses:
Research and development	8,459	6,910	24,758	20,366
General and administrative	3,134	3,653	9,238	8,350
Total operating expenses	11,593	10,563	33,996	28,716
Loss from operations	(10,935)	(9,966)	(31,722)	(27,730)
Other income (expense):
Change in fair value of derivative liabilities		(2)		1,547
Interest income and other income (expense), net	472	124	659	165
Total other income (expense), net	472	122	659	1,712
Net loss	(10,463)	(9,844)	(31,063)	(26,018)
Less: Net loss attributable to non-controlling interest		(8)		(1,137)
Net loss attributable to Spero Therapeutics, Inc.	(10,463)	(9,836)	(31,063)	(24,881)
Cumulative dividends on redeemable convertible preferred shares		(2,052)		(5,313)
Accretion of redeemable 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)
Net loss per share attributable to common stockholders per share, basic and diluted	$ (0.60)	$ (36.02)	$ (2.01)	$ (93.96)
Weighted average common shares outstanding, basic and diluted:	17,471,462	335,285	15,417,087	333,402
Comprehensive loss:
Net loss	$ (10,463)	$ (9,844)	$ (31,063)	$ (26,018)
Other comprehensive gain (loss):
Unrealized gain (loss) on marketable securities	(8)		(23)
Total other comprehensive gain (loss)	(8)		(23)
Total comprehensive loss	$ (10,471)	$ (9,844)	$ (31,086)	$ (26,018)