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Summary of Significant Accounting Policies - Schedule of Impacts to Accumulated Other Comprehensive Loss and Accumulated Deficit Upon Adoption of Guidance (Detail) - USD ($) $ in Thousands | Sep. 30, 2018 | Jan. 01, 2018 | Dec. 31, 2017
--- | --- | --- | ---
Accounting Policy [Line Items] |  |  |
Accumulated other comprehensive loss | $ (2,570) |  | $ (1,795)
Accumulated deficit | $ (736,780) |  | $ (630,657)
ASU 2016-01 [Member] |  |  |
Accounting Policy [Line Items] |  |  |
Accumulated other comprehensive loss |  | $ (3,045) |
Accumulated deficit |  | (629,407) |
Impact of change in accounting principle upon adoption of ASU 2016-01 [Member] | ASU 2016-01 [Member] |  |  |
Accounting Policy [Line Items] |  |  |
Accumulated other comprehensive loss |  | (1,250) |
Accumulated deficit |  | $ 1,250 |

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- Definition Accumulated change in equity from transactions and other events and circumstances from non-owner sources, net of tax effect, at period end. Excludes Net Income (Loss), and accumulated changes in equity from transactions resulting from investments by owners and distributions to owners. Includes foreign currency translation items, certain pension adjustments, unrealized gains and losses on certain investments in debt and equity securities, other than temporary impairment (OTTI) losses related to factors other than credit losses on available-for-sale and held-to-maturity debt securities that an entity does not intend to sell and it is not more likely than not that the entity will be required to sell before recovery of the amortized cost basis, as well as changes in the fair value of derivatives related to the effective portion of a designated cash flow hedge. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 220 -SubTopic 10 -Section 45 -Paragraph 11 -URI http://asc.fasb.org/extlink&oid=114867106&loc=d3e637-108580 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 220 -SubTopic 10 -Section 45 -Paragraph 14A -URI http://asc.fasb.org/extlink&oid=114867106&loc=SL7669686-108580 Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 220 -SubTopic 10 -Section 45 -Paragraph 14 -URI http://asc.fasb.org/extlink&oid=114867106&loc=d3e681-108580 + Details
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