Government Contracts - Additional Information (Detail)	1 Months Ended							3 Months Ended
	Jan. 31, 2019	Jul. 31, 2018 USD ($) Option	Feb. 28, 2018 USD ($)	Apr. 30, 2017 USD ($)	Feb. 28, 2017 USD ($)	Sep. 30, 2016 USD ($)	Jun. 30, 2016 USD ($) Option	Mar. 31, 2019 USD ($) Option	Mar. 31, 2018 USD ($)	Mar. 12, 2018 USD ($)
Government Contracts [Line Items]
Grant revenue								$ 7,718,000	$ 1,153,000
BARDA [Member]
Government Contracts [Line Items]
Potential contract amount awarded		$ 44,200,000
Option exercise | Option		2
BARDA [Member] | Base Period Contracts [Member]
Government Contracts [Line Items]
Potential amount initial funding awarded		$ 15,700,000
Contract term		3 years
BARDA [Member] | Grant [Member]
Government Contracts [Line Items]
Grant revenue								3,600,000
BARDA [Member] | First Option [Member]
Government Contracts [Line Items]
Potential grant amount awarded		$ 13,600,000
BARDA [Member] | Second Option [member]
Government Contracts [Line Items]
Potential grant amount awarded		$ 14,900,000
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									100,000
U.S. Department of Defense [Member] | Grant [Member] | Maximum [Member]
Government Contracts [Line Items]
Grant revenue								100,000
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							$ 6,300,000
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] | Spero Cantab [Member] | Pro Bono Bio PLC [Member] | Cantab Agreements [Member] | Research and Development Expenses [Member]
Government Contracts [Line Items]
Expense associated with amounts payable under agreement								100,000	100,000
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								$ 300,000	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								$ 5,700,000
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