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Warrants and Derivative Liabilities - Schedule of Assumptions Used to Calculate the Fair Value of Exchanged Warrants (Detail) - November 2014 Warrant $ in Millions	Sep. 30, 2018 USD ($)	Jun. 30, 2018 USD ($)	Mar. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Sep. 30, 2017 USD ($)	Jun. 30, 2017 USD ($)	Mar. 31, 2017 USD ($)
Derivative [Line Items]
Fair value	$ 1.3	$ 1.6	$ 1.1	$ 0.4	$ 0.8	$ 0.9	$ 1.8
Risk-free interest rate
Derivative [Line Items]
Measurement inputs	0.0262	0.0240	0.0220	0.0187	0.0149	0.0144	0.0141
Expected annual dividend yield
Derivative [Line Items]
Measurement inputs	0	0	0	0	0	0	0
Expected volatility
Derivative [Line Items]
Measurement inputs	0.6366	0.6740	0.6586	0.6586	0.6564	0.6721	0.6653
Term (years)
Derivative [Line Items]
Term (years)	1 year 1 month 13 days	1 year 4 months 13 days	1 year 7 months 13 days	1 year 10 months 13 days	2 years 1 month 13 days	2 years 4 months 13 days	2 years 7 months 13 days

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Value of outstanding derivative securities that permit the holder the right to purchase securities (usually equity) from the issuer at a specified price.

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Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 235
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.4-08.(i))
-URI http://asc.fasb.org/extlink&oid=26873400&loc=d3e23780-122690

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Value of input used to measure outstanding warrant and right embodying unconditional obligation requiring redemption by transferring asset at specified or determinable date or upon event certain to occur.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (bbb)(2)
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+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
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