CONDENSED CONSOLIDATED STATEMENTS OF STOCKHOLDERS' EQUITY - USD ($) $ in Thousands	Total	Series A and B Convertible Preferred Stock [Member]	Common Stock [Member]	Additional Paid-in Capital [Member]	Accumulated Deficit [Member]	Accumulated Other Comprehensive Income (Loss) [Member]	Spero Therapeutics, Inc. Stockholders' Equity [Member]	Non-controlling Interests [Member]
Balances at Dec. 31, 2017	$ 84,957		$ 14	$ 181,428	$ (96,840)		$ 84,602	$ 355
Balances, Shares at Dec. 31, 2017			14,369,182
Share-based compensation expense	614			614			614
Unrealized gain (loss) on available-for-sale securities	(29)					$ (29)	(29)
Net loss	(10,644)				(10,644)		(10,644)
Balances at Mar. 31, 2018	74,898		$ 14	182,042	(107,484)	(29)	74,543	355
Balances, Shares at Mar. 31, 2018			14,369,182
Balances at Dec. 31, 2018	115,855		$ 17	254,013	(138,502)	(28)	115,500	355
Balances, Shares at Dec. 31, 2018		3,220	17,205,962
Issuance of common stock upon the exercise of stock options	59			59			59
Issuance of common stock upon the exercise of stock options, shares			10,014
Issuance of common stock, net of issuance costs	1,525			1,525			1,525
Issuance of common stock, net of issuance costs, shares			119,090
Share-based compensation expense	933			933			933
Unrealized gain (loss) on available-for-sale securities	29					29	29
Net loss	(5,072)				(5,072)		(5,072)
Balances at Mar. 31, 2019	$ 113,329		$ 17	$ 256,530	$ (143,574)	$ 1	$ 112,974	$ 355
Balances, Shares at Mar. 31, 2019		3,220	17,335,066