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Significant Accounting Policies - Additional Information (Detail) $ in Millions | 9 Months Ended | 12 Months Ended |
--- | --- | --- | ---
Sep. 30, 2018 Segment | Dec. 31, 2017 USD ($) | Dec. 31, 2016 USD ($) | Jan. 01, 2016 USD ($)
Accounting Policy [Line Items] |  |  |  |
Number of operating segment | Segment | 1 |  |  |
Short term investments Maturity | 12 months |  |  |
Long term Investments Maturity | 12 months |  |  |
U.S. federal statutory income tax rate | 21.00% | 35.00% |  |
ASU 2014-09 [Member] | Accumulated Deficit [Member] |  |  |  |
Accounting Policy [Line Items] |  |  |  |
Increase in accumulative deficit |  |  |  | $ 43.7
New Revenue Standards [Member] | Revenue [Member] |  |  |  |
Accounting Policy [Line Items] |  |  |  |
Increase (reduction) in revenue |  | $ 5.3 | $ 3.6 |

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- Definition Percentage of domestic federal statutory tax rate applicable to pretax income (loss). + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 740 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SAB TOPIC 6.I) -URI http://asc.fasb.org/extlink&oid=34349781&loc=d3e330036-122817 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 740 -SubTopic 10 -Section 50 -Paragraph 12 -URI http://asc.fasb.org/extlink&oid=84230637&loc=d3e32687-109319 Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 740 -SubTopic 10 -Section 50 -Paragraph 13 -URI http://asc.fasb.org/extlink&oid=84230637&loc=d3e32698-109319 + Details
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