Income Taxes - Additional Information (Detail) - USD ($)	3 Months Ended	9 Months Ended	12 Months Ended
Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Income Tax Disclosure [Abstract]
Provision for federal income taxes	$ 0		$ 0
Income tax expense (benefit) for foreign taxes	5,000	$ 37,000	$ 400,000	$ 100,000
Federal statutory rate			21.00%		35.00%
Decrease of net deferred tax assets	$ 0				$ 7,200,000

Tax cuts and jobs act of 2017 change in tax rate deferred tax assets.

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- Definition

Percentage of domestic federal statutory tax rate applicable to pretax income (loss).

+ References

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-Publisher FASB
-Name Accounting Standards Codification
-Topic 740
-SubTopic 10
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- Definition

Amount of current and deferred federal income tax expense (benefit) attributable to income (loss) from continuing operations.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 235
-SubTopic 10
-Section S99
-Paragraph 1
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- Definition

Amount of current and deferred foreign income tax expense (benefit) attributable to income (loss) from continuing operations.

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