Amount of increase (decrease) in cash, cash equivalents, and cash and cash equivalents restricted to withdrawal or usage; including effect from exchange rate change. Cash includes, but is not limited to, currency on hand, demand deposits with banks or financial institutions, and other accounts with general characteristics of demand deposits. Cash equivalents include, but are not limited to, short-term, highly liquid investments that are both readily convertible to known amounts of cash and so near their maturity that they present insignificant risk of changes in value because of changes in interest rates.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 830
-SubTopic 230
-Section 45
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=98513438&loc=d3e33268-110906

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 24
-URI http://asc.fasb.org/extlink&oid=98514028&loc=d3e3521-108585

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

Summary of Significant Accounting Policies - Additional Information (Detail) - USD ($)	3 Months Ended		12 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017	Dec. 31, 2016
Summary Of Significant Accounting Policies [Line Items]
Cash equivalents				$ 0
Accounting Standards Update 2016-18 [Member]
Summary Of Significant Accounting Policies [Line Items]
Cash, cash equivalents increased due to inclusion		$ 50,000
Spero Potentiator Australia Pty Limited [Member] \| Research and Development Expenses [Member]
Summary Of Significant Accounting Policies [Line Items]
Percentage of refundable tax incentive	43.50%		43.50%