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Investments, Carried at Cost - Details of Investments Carried at Cost (Detail) - USD ($) $ in Thousands | Sep. 30, 2019 | Dec. 31, 2018
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Investment [Line Items] |  |
Investments, carried at cost | $ 28,080 | $ 28,080
AdvisorEngine [Member] | Convertible Preferred Stock [Member] |  |
Investment [Line Items] |  |
Investments, carried at cost | 25,000 | 25,000
Thesys Group, Inc [Member] |  |
Investment [Line Items] |  |
Investments, carried at cost | $ 3,080 | $ 3,080

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- Definition Amount, after adjustment, of cost-method investment. Adjustments include, but are not limited to, dividends received in excess of earnings after date of investment that are considered a return of investment and other than temporary impairments. + References No definition available. + Details
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Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

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- Definition Line items represent reporting concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table. + References No definition available. + Details
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Namespace Prefix: | wetf_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Details
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Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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- Details
Name: | us-gaap_InvestmentTypeAxis=us-gaap_ConvertiblePreferredStockMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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- Details
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Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |