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Goodwill and Intangible Assets - Additional Information (Detail) $ in Thousands, $ in Thousands |  |  | 3 Months Ended | 9 Months Ended |
--- | --- | --- | --- | --- | ---
Apr. 30, 2018 USD ($) | Apr. 11, 2018 USD ($) | Dec. 31, 2017 USD ($) Fund | Dec. 31, 2017 CAD ($) Fund | Sep. 30, 2018 USD ($) | Jan. 01, 2016 USD ($)
Indefinite-lived Intangible Assets [Line Items] |  |  |  |  |  |
Intangible asset related to its customary advisory agreement |  |  | $ 12,085 |  | $ 613,274 |
U.S. Business segment [Member] |  |  |  |  |  |
Indefinite-lived Intangible Assets [Line Items] |  |  |  |  |  |
Goodwill, impairment loss | $ 0 |  |  |  |  |
Questrade ETFs [Member] |  |  |  |  |  |
Indefinite-lived Intangible Assets [Line Items] |  |  |  |  |  |
Intangible asset related to its customary advisory agreement |  |  | $ 2,132 |  | 2,074 |
Questrade ETFs [Member] | WisdomTree Asset Management Canada, Inc [Member] |  |  |  |  |  |
Indefinite-lived Intangible Assets [Line Items] |  |  |  |  |  |
Number of ETF acquired by company | Fund |  |  | 8 | 8 |  |
Assets under management |  |  | $ 77,403 | $ 99,108 |  |
Business acquisition, cash paid |  |  | 2,132 | $ 2,675 |  |
GreenHaven Commodity Services, LLC [Member] |  |  |  |  |  |
Indefinite-lived Intangible Assets [Line Items] |  |  |  |  |  |
Intangible asset related to its customary advisory agreement |  |  | $ 9,953 |  | 9,953 | $ 9,953
GreenHaven Commodity Services, LLC [Member] | U.S. Business segment [Member] |  |  |  |  |  |
Indefinite-lived Intangible Assets [Line Items] |  |  |  |  |  |
Goodwill acquired |  |  |  |  | $ 1,799 |
Acquisition of ETFS Business [Member] |  |  |  |  |  |
Indefinite-lived Intangible Assets [Line Items] |  |  |  |  |  |
Acquisition completion date |  |  |  |  | Apr. 11, 2018 |
Goodwill acquired |  |  |  |  | $ 84,057 |
Intangible asset related to its customary advisory agreement |  |  |  |  | $ 601,247 |
Weighted average cost of capital percentage |  | 11.60% |  |  |  |
Business acquisition, cash paid |  | $ 253,000 |  |  |  |
Acquisition of ETFS Business [Member] | Minimum [Member] |  |  |  |  |  |
Indefinite-lived Intangible Assets [Line Items] |  |  |  |  |  |
Revenue growth multiple |  | 3.00% |  |  |  |
Acquisition of ETFS Business [Member] | Maximum [Member] |  |  |  |  |  |
Indefinite-lived Intangible Assets [Line Items] |  |  |  |  |  |
Revenue growth multiple |  | 4.00% |  |  |  |

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