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Capital Stock and Derivative Instruments - Schedule of Vesting Shares Subject to Repurchase (Detail) - Restricted shares subject to repurchase [Member] - shares | Mar. 31, 2018 | Dec. 31, 2017 | Dec. 31, 2016
--- | --- | --- | ---
Amortization Expense [Line Items] |  |  |
Unvested shares | 3,834,512 | 4,053,640 | 4,039,591
Total unvested shares - subject to vesting schedule | 278,956 |  |
Unvested shares - event dependent | 3,555,556 |  |
1st quarter 2019 [Member] |  |  |
Amortization Expense [Line Items] |  |  |
Unvested shares | 167,848 |  |
1st quarter 2020 [Member] |  |  |
Amortization Expense [Line Items] |  |  |
Unvested shares | 111,108 |  |

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- Definition The number of non-vested equity-based payment instruments, excluding stock (or unit) options, that validly exist and are outstanding as of the balance sheet date. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c)(2)(i)-(ii) -URI http://asc.fasb.org/extlink&oid=96867065&loc=d3e5070-113901 + Details
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- Definition Amortization expense. + References No definition available. + Details
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- Definition Share based compensation arrangement by share based payment award equity instruments other than options nonvested number event dependent. + References No definition available. + Details
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- Definition Share based compensation arrangement by share based payment award equity instruments other than options nonvested number subject to vesting schedule. + References No definition available. + Details
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