CONSOLIDATED STATEMENTS OF CONVERTIBLE PREFERRED SHARES AND STOCKHOLDERS' EQUITY - USD ($) $ in Thousands	Total	Series A Preferred Stock [Member]	Series A and B Convertible Preferred Stock [Member]	Series A and B Convertible Preferred Stock [Member] Series A Preferred Stock [Member]	Series A and B Convertible Preferred Stock [Member] Series B Preferred Stock [Member]	Common Stock [Member]	Common Stock [Member] Series A Preferred Stock [Member]	Common Stock [Member] Series B Preferred Stock [Member]	Accumulated Other Comprehensive Income (Loss) [Member]	Additional Paid-in Capital [Member]	Additional Paid-in Capital [Member] Spero Gyrase, Inc. [Member]	Additional Paid-in Capital [Member] Series A Preferred Stock [Member]	Additional Paid-in Capital [Member] Series B Preferred Stock [Member]	Accumulated Deficit [Member]	Spero Therapeutics, Inc. Stockholders' Equity (Deficit) [Member]	Spero Therapeutics, Inc. Stockholders' Equity (Deficit) [Member] Spero Gyrase, Inc. [Member]	Spero Therapeutics, Inc. Stockholders' Equity (Deficit) [Member] Series A Preferred Stock [Member]	Non-controlling Interests [Member]	Non-controlling Interests [Member] Spero Gyrase, Inc. [Member]
Balances at Dec. 31, 2017	$ 84,957					$ 14				$ 181,428				$ (96,840)	$ 84,602			$ 355
Balance, Shares at Dec. 31, 2017						14,369,182
Issuance of common stock upon the exercise of stock options	335									335					335
Issuance of common stock upon the exercise of stock options, shares						56,780
Issuance of common and Series A preferred stock in public offering, net of issuance costs of $996		$ 69,504					$ 4					$ 69,500					$ 69,504
Issuance of common and Series A preferred stock in public offering, net of issuance costs of $996, Shares				2,220			3,780,000
Issuance of preferred stock in exchange for common stock								$ (1)					$ 1
Issuance of Series B preferred stock in exchange for common stock, shares					1,000			(1,000,000)
Share-based compensation expense	2,749									2,749					2,749
Unrealized gain (loss) on available-for-sale securities	(28)								$ (28)						(28)
Net loss	(41,662)													(41,662)	(41,662)
Balances at Dec. 31, 2018	115,855					$ 17			(28)	254,013				(138,502)	115,500			$ 355
Balance, shares at Dec. 31, 2018			3,220			17,205,962
Issuance of common stock upon the exercise of stock options	$ 508									508					508
Issuance of common stock upon the exercise of stock options, shares	78,610					78,610
Issuance of common stock, net of issuance costs of $474	$ 15,316					$ 1				15,315					15,316
Issuance of stock, shares						1,406,123
Issuance of Common Stock, conversion of Preferred Stock to Common Stock						$ 1				(1)
Issuance of Common Stock, conversion of Preferred Stock to Common Stock, shares			(500)			500,000
Share-based compensation expense	3,776									3,776					3,776
Purchase of non-controlling interest											$ 355					$ 355			$ (355)
Unrealized gain (loss) on available-for-sale securities	44								44						44
Net loss	(60,925)													(60,925)	(60,925)
Balances at Dec. 31, 2019	$ 74,574					$ 19			$ 16	$ 273,966				$ (199,427)	$ 74,574
Balance, shares at Dec. 31, 2019			2,720			19,190,695