Fair Value of Financial Assets and Liabilities - Additional Information (Detail) - USD ($)		1 Months Ended									12 Months Ended
	Jul. 01, 2015	Mar. 31, 2017	Dec. 31, 2016	Aug. 31, 2016	Feb. 29, 2016	Jan. 31, 2016	Nov. 30, 2015	Jun. 30, 2015	Feb. 28, 2015	Apr. 30, 2014	Dec. 31, 2017	Dec. 31, 2016	Jun. 30, 2017	May 31, 2017	Mar. 31, 2016	Jan. 31, 2015	Dec. 31, 2014
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value assets level 1 to level 2 transfers			$ 0								$ 0	$ 0
Fair value assets level 2 to level 1 transfers			0								0	0
Fair value liabilities level 1 to level 2 transfers			0								0	0
Fair value liabilities level 2 to level 1 transfers			0								0	0
Transfer of financial asset into level 3 of fair value											0	0
Transfer of financial liabilities into level 3 of fair value											0	$ 0
Roche Collaboration Agreements [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Research and development agreement, potential investment										$ 2,000,000
Percentage of issuance price										90.00%
Investment Option [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative liabilities								$ 100,000									$ 200,000
Derivative liability								(100,000)
Spero Potentiator, Inc. [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Number of shares issued				2,160		2,160	2,736
Proceeds from issuing additional shares				$ 0		$ 0	$ 0
Fair value inputs probability of funding				100.00%
Spero Potentiator, Inc. [Member] | Northern Antibiotics Oy Ltd. [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Number of shares issued				2,160		2,160	2,736		996
Amount of shares issued				$ 1,000,000		$ 1,000,000	$ 1,500,000
Spero Potentiator, Inc. [Member] | Anti- dilution Rights [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative liabilities									$ 2,400,000
Spero Potentiator, Inc. [Member] | Anti- dilution Rights [Member] | Northern Antibiotics Oy Ltd. [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Proceeds from issuing additional shares				$ 0		0	$ 0		$ 0
Spero Europe, Ltd. [Member] | Anti- dilution Rights [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative liabilities						$ 200,000								$ 0
Derivative liability											$ (200,000)
Fair value inputs probability of funding											0.00%	100.00%
Spero Gyrase, Inc. [Member] | Anti- dilution Rights [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative liabilities											$ 200,000		$ 200,000		$ 1,600,000
Derivative liability											$ (1,400,000)
Fair value inputs probability of funding											0.00%	100.00%
Class A Tranche Rights[Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative liabilities								2,400,000
Derivative liability					$ (2,400,000)
Class B Tranche Rights [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative liabilities			300,000		$ 900,000							$ 300,000
Derivative liability			(600,000)
January 2015 Bridge Units [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative liabilities								$ 900,000
Derivative liability	$ (1,400,000)
January 2015 Bridge Units [Member] | Contingent prepayment option [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative liabilities																$ 2,300,000
Class A Preferred Unit [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Preferred units discount percentage	10.00%							20.00%
Conversion price of redeemable preferred share	$ 3.51							$ 3.90
December 2016 Bridge Units [Member] | Contingent prepayment option [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative liabilities			$ 900,000									$ 900,000
December 2016 Bridge Units [Member] | Contingent prepayment option [Member] | Maximum [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative liability		$ 100,000