CONSOLIDATED STATEMENTS OF OPERATIONS AND COMPREHENSIVE LOSS - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Statement [Abstract]
Grant revenue	$ 1,979	$ 335
Operating expenses:
Research and development	32,869	26,333	$ 11,125
General and administrative	10,840	7,223	2,202
Total operating expenses	43,709	33,556	13,327
Loss from operations	(41,730)	(33,221)	(13,327)
Other income (expense):
Change in fair value of derivative liabilities	1,541	580	174
Interest income and other income (expense), net	303
Total other income (expense), net	1,844	580	174
Net loss and comprehensive loss	(39,886)	(32,641)	(13,153)
Less: Net loss attributable to non-controlling interest	(1,143)	(7,150)	(2,999)
Net loss attributable to Spero Therapeutics, Inc.	(38,743)	(25,491)	(10,154)
Cumulative dividends on redeemable convertible preferred shares	(6,146)	(3,441)	(932)
Accretion of redeemable bridge units and redeemable convertible preferred shares to redemption value	(1,208)	(996)	(2,341)
Net loss attributable to common shareholders of Spero Therapeutics, Inc.	$ (46,097)	$ (29,928)	$ (13,427)
Net loss per share attributable to common shareholders per share, basic and diluted	$ (17.82)	$ (95.87)	$ (53.11)
Weighted average shares outstanding, basic and diluted:	2,586,865	312,169	252,807