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Investments and Fair Value Measurements - Schedule of Investments (Detail) (USD $) In Thousands, unless otherwise specified | Dec. 31, 2014 | Dec. 31, 2013
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Schedule of Held-to-maturity Securities [Line Items] |  |
Held-to-Maturity debt instruments | $ 13,990us-gaap_HeldToMaturitySecuritiesAmortizedCostBeforeOtherThanTemporaryImpairment | $ 11,748us-gaap_HeldToMaturitySecuritiesAmortizedCostBeforeOtherThanTemporaryImpairment
Federal Agency [Member] |  |
Schedule of Held-to-maturity Securities [Line Items] |  |
Held-to-Maturity debt instruments | $ 13,990us-gaap_HeldToMaturitySecuritiesAmortizedCostBeforeOtherThanTemporaryImpairment / us-gaap_DebtSecurityAxis = wetf_FederalAgencyMember | $ 11,748us-gaap_HeldToMaturitySecuritiesAmortizedCostBeforeOtherThanTemporaryImpairment / us-gaap_DebtSecurityAxis = wetf_FederalAgencyMember

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- Definition Amount before other than temporary impairment (OTTI) accretion, of investments in debt securities classified as held-to-maturity. + References No definition available. + Details
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