Collaboration and License Agreements - Additional Information (Detail) £ in Millions		1 Months Ended								3 Months Ended		12 Months Ended
	Jan. 04, 2019 USD ($)	Nov. 30, 2017 USD ($)	Jun. 30, 2017 USD ($)	Jun. 30, 2016 USD ($)	Jun. 30, 2016 GBP (£)	May 31, 2016 USD ($)	Mar. 31, 2016 USD ($)	Nov. 30, 2015 USD ($) shares	Feb. 28, 2015 USD ($) shares	Mar. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	Dec. 31, 2018 USD ($)	Oct. 31, 2017 USD ($)
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Common stock issued, aggregate fair value										$ 1,525,000
Proceeds from the issuance of common stock, net of issuance costs										1,525,000
Revenue recognized										3,807,000
Aviragen Agreement [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Potential milestone payment upon achievement of specified clinical, regulatory and commercial milestones							$ 12,000,000
Cantab Related Agreements [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Potential milestone payment upon achievement of specified clinical and regulatory milestones				$ 5,800,000
Potential milestone payment upon achievement of specified commercial milestone					£ 5.0					$ 6,500,000		$ 6,400,000
License agreement research and development expense related to achievement of regulatory milestones												200,000
Contract termination period				10 years	10 years
Vertex License Agreement [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
License agreement research and development expense related to achievement of regulatory milestones												$ 200,000
Contract termination period										10 years
Nonrefundable upfront payments						$ 500,000
Potential milestone payment upon achievement of specified clinical, regulatory and commercial milestones						$ 81,100,000
Contract termination period if no material development or commercialization occurs										1 year
Meiji License Agreement [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Potential milestone payment upon achievement of specified clinical and regulatory milestones			$ 3,000,000
Nonrefundable upfront payments			600,000
Potential milestone payments upon achievement of specified condition													$ 1,000,000
License agreement fixed assets related payments											$ 1,600,000
Meiji License Agreement [Member] | Maximum [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Sublicense fee payable to counter party			7,500,000
Northern License Agreement [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Potential milestone payment upon achievement of specified clinical, regulatory and other milestones			7,000,000
Potential milestone payment upon closing of initial public offering			2,500,000
Northern License Agreement [Member] | Spero Potentiator, Inc. [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Common stock shares issued | shares								2,736
Proceeds from the issuance of common stock, net of issuance costs								$ 0
Northern License Agreement [Member] | Spero Potentiator, Inc. [Member] | Northern Antibiotics Oy Ltd. [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Common stock shares issued | shares									996
Common stock issued, aggregate fair value									$ 1,100,000
Northern License Agreement [Member] | Spero Potentiator, Inc. [Member] | Northern Antibiotics Oy Ltd. [Member] | Anti- dilution Rights [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Ownership percentage									49.90%
Proceeds from the issuance of common stock, net of issuance costs									$ 0
Northern Exchange Agreement [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Potential milestone payment upon closing of initial public offering			$ 100,000
Northern License And Exchange Agreement [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Potential milestone payment upon closing of initial public offering		$ 2,600,000
Everest License Agreement [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Period of option granted on an agreement										12 months
Upfront payment received	$ 3,000,000
Potential milestone payments upon completion and delivery of results of a clinical study	2,000,000
Maximum additional milestone payments receivable	55,000,000
Upfront License Fee										$ 2,000,000
Transaction price allocated to the performance obligations										5,000,000
Performance obligations exclusive option to negotiate license										1,000,000
Performance obligations related to research and development services milestone										2,000,000
Transaction price allocated to performance obligation that are unsatisfied										1,200,000
Everest License Agreement [Member] | Prepaid Expenses and Other Current Assets [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Contract assets										800,000
Everest License Agreement [Member] | SPR 206 [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Upfront payment received	2,000,000
Everest License Agreement [Member] | SPR 741.
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Upfront payment received	1,000,000
Everest License Agreement [Member] | Collaboration Revenue [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Revenue recognized										$ 3,800,000
Everest License Agreement [Member] | Maximum [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Future milestone payments	$ 1,500,000
Agreement termination period upon written notice	180 days
Everest License Agreement [Member] | Minimum [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Agreement termination period upon written notice	90 days