Fair Value of Financial Assets and Liabilities - Additional Information (Detail) - USD ($)				1 Months Ended					3 Months Ended				6 Months Ended	9 Months Ended
	Sep. 30, 2017	Dec. 31, 2016	Nov. 30, 2016	Dec. 31, 2016	Aug. 31, 2016	Feb. 29, 2016	Jan. 31, 2016	Feb. 28, 2015	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Sep. 30, 2016	Jun. 30, 2017	Sep. 30, 2017	Sep. 30, 2016	May 31, 2017	Mar. 31, 2016	Dec. 31, 2015
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		$ 0	$ 0
Fair value assets level 2 to level 1 transfers	0								0			0		0	0
Fair value liabilities level 1 to level 2 transfers	0								0			0		0	0
Fair value liabilities level 2 to level 1 transfers	0								0			0		0	0
Transfer of financial asset into level 3 of fair value									0			0		0	0
Transfer of financial liabilities into level 3 of fair value									0			$ 0		0	$ 0
Contingent prepayment option [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative liabilities		$ 900,000		$ 900,000
Contingent prepayment option [Member] | Maximum [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Increase (decrease) derivative liabilities											$ 100,000
Class A Tranche Rights[Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative liabilities																		$ 2,400,000
Increase (decrease) derivative liabilities						$ (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		$ 300,000		$ 900,000
Increase (decrease) derivative liabilities			$ (600,000)
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
Proceeds from issuing additional shares					$ 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	996
Amount of shares issued					$ 1,000,000		$ 1,000,000							1,000,000
Spero Potentiator, Inc. [Member] | Anti- dilution Rights [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative liabilities																		$ 1,000,000
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
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
Increase (decrease) derivative liabilities													$ (200,000)
Fair value inputs probability of funding				100.00%						0.00%	0.00%
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	$ 200,000			200,000	$ 200,000			$ 1,600,000
Increase (decrease) derivative liabilities													$ (1,400,000)
Fair value inputs probability of funding	0.00%	100.00%