Government Contracts - Additional Information (Detail)		1 Months Ended				3 Months Ended		12 Months Ended	24 Months Ended
	Jul. 01, 2019 USD ($)	Jul. 31, 2018 USD ($)	Feb. 28, 2018 USD ($)	Feb. 28, 2017 USD ($)	Jun. 30, 2016 USD ($) Option	Mar. 31, 2021 USD ($) Option	Mar. 31, 2020 USD ($)	Dec. 31, 2020	Feb. 28, 2019	Jan. 31, 2020 USD ($)	May 31, 2019 USD ($)
Government Contracts [Line Items]
Contract term									2 years
Grant revenue						$ 7,300,000	$ 1,701,000
BARDA and DTRA [Member]
Government Contracts [Line Items]
Maximum potential funding from government contract						56,800,000
BARDA and Speros Achievement of Specified Milestones [Member]
Government Contracts [Line Items]
Potential grant amount exercised						12,700,000
Grant [Member] | DoD, CDMRP and JWMRP [Member]
Government Contracts [Line Items]
Grant revenue						600,000
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									$ 18,200,000
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						6,300,000	1,200,000
BARDA [Member] | First Option [Member]
Government Contracts [Line Items]
Potential amount additional funding awarded										$ 15,900,000
Potential amount increase in committed funding										34,100,000
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
Contact extension term								Feb. 28, 2020
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
Contact extension term						Jun. 30, 2021
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]
Government Contracts [Line Items]
Grant revenue						$ 0
Niaid [Member] | Grant [Member] | Spero Cantab [Member] | Cantab Anti Infectives Ltd. [Member] | Cantab Agreements [Member]
Government Contracts [Line Items]
Grant revenue							$ 300,000
Niaid [Member] | Grant [Member] | Maximum [Member]
Government Contracts [Line Items]
Grant revenue						100,000
Niaid [Member] | Grant [Member] | Maximum [Member] | Spero Cantab [Member] | Cantab Anti Infectives Ltd. [Member] | Cantab Agreements [Member]
Government Contracts [Line Items]
Grant revenue						$ 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