Government Contracts - Additional Information (Detail)		1 Months Ended							12 Months Ended
	Jul. 01, 2019 USD ($)	Jan. 31, 2019	Jul. 31, 2018 USD ($)	Feb. 28, 2018 USD ($)	Apr. 30, 2017 USD ($)	Feb. 28, 2017 USD ($)	Sep. 30, 2016 USD ($)	Jun. 30, 2016 USD ($) Option	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($) Option	Jan. 31, 2020 USD ($)	May 31, 2019 USD ($)	Mar. 12, 2018 USD ($)
Government Contracts [Line Items]
Grant revenue									$ 18,147,000	$ 3,966,000
Grant [Member] | DoD, CDMRP and JWMRP [Member] | Maximum [Member]
Government Contracts [Line Items]
Grant revenue									100,000
SPR206 [Member] | DoD, CDMRP and JWMRP [Member]
Government Contracts [Line Items]
Award received for clinical development	$ 5,900,000
Non-dilutive funding period	4 years
BARDA [Member]
Government Contracts [Line Items]
Potential contract amount awarded			$ 44,200,000
BARDA [Member] | Base Period Contracts [Member]
Government Contracts [Line Items]
Potential amount initial funding awarded			$ 15,700,000									$ 18,100,000
Potential amount additional funding awarded												2,500,000
Overall potential award increased												$ 46,800,000
Contract term			3 years
BARDA [Member] | Grant [Member]
Government Contracts [Line Items]
Grant revenue									12,100,000	1,400,000
BARDA [Member] | Subsequent Event [Member] | First Option [Member]
Government Contracts [Line Items]
Potential amount additional funding awarded											$ 15,900,000
Potential amount increase in committed funding											34,100,000
BARDA [Member] | Subsequent Event [Member] | Second Option [member]
Government Contracts [Line Items]
Potential grant amount awarded											$ 12,700,000
U.S. Department of Defense [Member]
Government Contracts [Line Items]
Contract term							2 years
Potential grant amount awarded							$ 1,500,000
U.S. Department of Defense [Member] | Grant [Member]
Government Contracts [Line Items]
Grant revenue									200,000	300,000
Niaid [Member]
Government Contracts [Line Items]
Contract term						12 months
Niaid [Member] | Spero Cantab [Member] | Cantab Anti Infectives Ltd. [Member] | Cantab Agreements [Member]
Government Contracts [Line Items]
Maximum potential funding from government contract								$ 6,500,000
Number of option period for funding from government contract | Option								3
Maximum received fund										1,300,000
Amount paid upfront as part of agreement										300,000
Niaid [Member] | Base Period Contracts [Member]
Government Contracts [Line Items]
Potential grant amount awarded						$ 600,000
Niaid [Member] | Grant [Member]
Government Contracts [Line Items]
Grant revenue									100,000	500,000
Niaid [Member] | Grant [Member] | Spero Cantab [Member] | Cantab Anti Infectives Ltd. [Member] | Cantab Agreements [Member]
Government Contracts [Line Items]
Grant revenue									1,000,000	$ 1,400,000
Niaid [Member] | Option Period Contracts [Member]
Government Contracts [Line Items]
Contract term				12 months
Potential grant amount awarded						$ 400,000
Potential grant amount exercised				$ 400,000
Contract term extended		12 months
Niaid [Member] | First Option [Member] | Spero Cantab [Member] | Cantab Anti Infectives Ltd. [Member] | Cantab Agreements [Member]
Government Contracts [Line Items]
Number of option period for funding from government contract | Option										2
Committed amount from government contract										$ 5,900,000
CARB-X [Member]
Government Contracts [Line Items]
Contract term					12 months
Potential grant amount awarded					$ 1,500,000
Potential grant amount exercised					$ 0
CARB-X [Member] | First Option [Member]
Government Contracts [Line Items]
Potential grant amount awarded													$ 400,000
CARB-X [Member] | Grant [Member]
Government Contracts [Line Items]
Grant revenue									$ 0	$ 500,000