Government Contracts - Additional Information (Detail)	1 Months Ended						3 Months Ended					12 Months Ended
	Feb. 28, 2018 USD ($)	Dec. 31, 2017 USD ($) Option	Apr. 30, 2017 USD ($)	Feb. 28, 2017 USD ($)	Sep. 30, 2016 USD ($)	Jun. 30, 2016 USD ($) Option	Dec. 31, 2017 USD ($)	Sep. 30, 2017 USD ($)	Jun. 30, 2017 USD ($)	Mar. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Dec. 31, 2017 USD ($) Option	Dec. 31, 2016 USD ($)	Mar. 12, 2018 USD ($)
Government Contracts [Line Items]
Grant revenue							$ 993,000	$ 597,000	$ 249,000	$ 140,000	$ 335,000	$ 1,979,000	$ 335,000
Revenue included in other receivables		$ 1,011,000					1,011,000				304,000	1,011,000	304,000
U.S. Department of Defense [Member]
Government Contracts [Line Items]
Contract term					2 years
Potential grant amount awarded					$ 1,500,000
Grant revenue												700,000	300,000
Revenue included in other receivables		100,000					100,000				$ 300,000	100,000	$ 300,000
Niaid [Member]
Government Contracts [Line Items]
Contract term				12 months
Grant revenue												400,000
Niaid [Member] | Spero Cantab [Member] | Cantab Anti Infectives Ltd. [Member] | Cantab Agreements [Member]
Government Contracts [Line Items]
Maximum potential funding from government contract						$ 5,700,000
Number of option period for funding from government contract | Option						3
Maximum received fund		$ 1,300,000										$ 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 | Option		2										2
Committed amount from government contract		$ 5,100,000										$ 5,100,000
Niaid [Member] | Maximum [Member] | Other Receivables [Member]
Government Contracts [Line Items]
Revenue invoiced but unpaid		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]
Potential grant amount awarded				$ 400,000
Niaid [Member] | Option Period Contracts [Member] | Subsequent Event [Member]
Government Contracts [Line Items]
Contract term	12 months
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												900,000
Revenue included in other receivables		$ 700,000					$ 700,000					$ 700,000
Potential grant amount exercised			$ 0
CARB-X [Member] | Subsequent Event [Member] | First Option [Member]
Government Contracts [Line Items]
Potential grant amount awarded														$ 400,000