Government Contracts - Additional Information (Detail)		1 Months Ended					12 Months Ended		24 Months Ended
	Jul. 01, 2019 USD ($)	May 31, 2021 USD ($) Option	Jul. 31, 2018 USD ($)	Feb. 28, 2018 USD ($)	Feb. 28, 2017 USD ($)	Jun. 30, 2016 USD ($) Option	Dec. 31, 2021 USD ($) Option	Dec. 31, 2020 USD ($)	Feb. 28, 2019	Jan. 19, 2022 USD ($)	Jan. 31, 2020 USD ($)	May 31, 2019 USD ($)
Government Contracts [Line Items]
Grant revenue							$ 18,256,000	$ 9,330,000
BARDA and DTRA [Member]
Government Contracts [Line Items]
Maximum potential funding from government contract							9,700,000
BARDA and Speros Achievement of Specified Milestones [Member]
Government Contracts [Line Items]
Potential grant amount exercised							12,700,000
Grant [Member] | Maximum [Member] | DoD, CDMRP and JWMRP [Member]
Government Contracts [Line Items]
Grant revenue							4,500,000	400,000
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							$ 59,700,000
Potential amount additional funding awarded										12,900,000
Overall potential award increased										$ 46,900,000
BARDA [Member] | Base Period Contracts [Member]
Government Contracts [Line Items]
Potential amount initial funding awarded			$ 15,700,000									$ 18,200,000
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							9,900,000	7,900,000
BARDA [Member] | First Option [Member]
Government Contracts [Line Items]
Potential amount additional funding awarded											$ 15,900,000
Potential amount increase in committed funding											34,000,000.0
BARDA [Member] | Second Option [member]
Government Contracts [Line Items]
Potential grant amount awarded											$ 12,700,000
DTRA [Member] | Maximum [Member]
Government Contracts [Line Items]
Potential amount additional funding awarded							10,000,000.0
Niaid [Member]
Government Contracts [Line Items]
Contract term					12 months				2 years
Grant revenue							0	100,000
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
Niaid [Member] | Base Period Contracts [Member]
Government Contracts [Line Items]
Potential grant amount awarded					$ 600,000
Niaid [Member] | Grant [Member] | Spero Cantab [Member] | Cantab Anti Infectives Ltd. [Member] | Cantab Agreements [Member]
Government Contracts [Line Items]
Grant revenue							$ 400,000	$ 700,000
Niaid [Member] | SPR206 [Member]
Government Contracts [Line Items]
Contract term		5 years
Award received for clinical development		$ 23,400,000
Number of option period for funding from government contract | Option		5
Committed amount from government contract		$ 2,100,000
Maximum received fund		$ 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
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