Non-Controlling Interests - Spero Potentiator - Additional Information (Detail) - USD ($)	1 Months Ended				3 Months Ended		9 Months Ended
	Jun. 30, 2017	Aug. 31, 2016	Jan. 31, 2016	Feb. 28, 2015	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016	Dec. 31, 2016
Noncontrolling Interest [Line Items]
Research and development expense					$ 6,910,000	$ 6,005,000	$ 20,366,000	$ 19,406,000
Non-controlling interest					$ 361,000		361,000		$ (3,808,000)
Spero Potentiator, Inc. [Member]
Noncontrolling Interest [Line Items]
Common stock shares issued		2,160	2,160
Proceeds from issuing additional shares		$ 0	$ 0
Non-controlling interest									$ (5,470,000)
Spero Potentiator, Inc. [Member] | Northern Antibiotics Oy Ltd. [Member]
Noncontrolling Interest [Line Items]
Common stock shares issued		2,160	2,160	996
Common stock issued, aggregate fair value				$ 1,100,000
Increase in non-controlling interest and reduction of derivative liability		$ 1,000,000	$ 1,000,000				$ 1,000,000
Cash payment for repurchase of shares	$ 1,000,000
Contingent consideration	100,000
Milestone payments	7,000,000
Cash payment for repurchase of shares	2,500,000
Non-controlling interest	6,400,000
Increase to accumulated deficit	$ 1,000,000
Spero Potentiator, Inc. [Member] | Northern Antibiotics Oy Ltd. [Member] | Anti- dilution Rights [Member]
Noncontrolling Interest [Line Items]
Ownership percentage				49.90%
Proceeds from issuing additional shares				$ 0
Spero Potentiator, Inc. [Member] | Northern Antibiotics Oy Ltd. [Member] | Anti- dilution Rights [Member] | Prior to IND [Member]
Noncontrolling Interest [Line Items]
Maximum amounts of investments for anti-dilution protection				5,000,000
Spero Potentiator, Inc. [Member] | Northern Antibiotics Oy Ltd. [Member] | Anti- dilution Rights [Member] | Post IND [Member]
Noncontrolling Interest [Line Items]
Maximum amounts of investments for anti-dilution protection				6,500,000
Spero Potentiator, Inc. [Member] | Northern Antibiotics Oy Ltd. [Member] | Licensing Agreements [Member]
Noncontrolling Interest [Line Items]
Research and development expense				$ 1,100,000