Government Contracts - Additional Information (Detail)	1 Months Ended							3 Months Ended		22 Months Ended
	Mar. 31, 2018 USD ($)	Feb. 28, 2018 USD ($)	Dec. 31, 2017 USD ($)	Apr. 30, 2017 USD ($)	Feb. 28, 2017 USD ($)	Sep. 30, 2016 USD ($)	Jun. 30, 2016 USD ($) Option	Mar. 31, 2018 USD ($)	Mar. 31, 2017 USD ($)	Mar. 31, 2018 USD ($) Option	Mar. 12, 2018 USD ($)
Government Contracts [Line Items]
Grant revenue								$ 1,153,000	$ 140,000
Amount payable under agreement	$ 982,000		$ 3,470,000					982,000		$ 982,000
U.S. Department of Defense [Member]
Government Contracts [Line Items]
Contract term						2 years
Potential grant amount awarded						$ 1,500,000
Grant revenue								100,000	$ 100,000
Niaid [Member]
Government Contracts [Line Items]
Contract term					12 months
Grant revenue								500,000
Niaid [Member] | Spero Cantab [Member] | Cantab Anti Infectives Ltd. [Member] | Cantab Agreements [Member]
Government Contracts [Line Items]
Maximum potential funding from government contract							$ 6,000,000
Number of option period for funding from government contract | Option							3
Maximum received fund			$ 1,300,000
Niaid [Member] | Spero Cantab [Member] | Cantab Anti Infectives Ltd. [Member] | Cantab Agreements [Member] | First Option [Member]
Government Contracts [Line Items]
Number of option period for funding from government contract committed to date | Option										2
Committed amount from government contract to date										$ 5,400,000
Niaid [Member] | Spero Cantab [Member] | Pro Bono Bio PLC [Member] | Cantab Agreements [Member]
Government Contracts [Line Items]
Amount paid upfront as part of agreement	300,000
Amount payable under agreement	$ 100,000							100,000		$ 100,000
Niaid [Member] | Base Period Contracts [Member]
Government Contracts [Line Items]
Potential grant amount awarded					$ 600,000
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
CARB-X [Member]
Government Contracts [Line Items]
Contract term				12 months
Potential grant amount awarded				$ 1,500,000
Grant revenue								$ 500,000
Potential grant amount exercised				$ 0
CARB-X [Member] | First Option [Member]
Government Contracts [Line Items]
Potential grant amount awarded											$ 400,000