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Related Party Transactions - Class of Shares (Details) - Tradeweb Markets LLC - USD ($) | 9 Months Ended | 12 Months Ended |  |
--- | --- | --- | --- | ---
Sep. 30, 2018 | Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2014 | Dec. 31, 2018 | Dec. 31, 2015
Successor |  |  |  |  |  |
Related Party Transactions |  |  |  |  |  |
Members' equity | $ 4,562,154,000 |  |  |  | $ 4,572,334,000 |
Employee equity compensation payable |  |  |  |  | 24,187,000 |
Successor | Banks | Class P-1(A) Shares |  |  |  |  |  |
Related Party Transactions |  |  |  |  |  |
Members' equity |  |  |  |  | 150,495,000 |
Successor | Employees | Class P-1(C) Shares |  |  |  |  |  |
Related Party Transactions |  |  |  |  |  |
Employee equity compensation payable |  |  |  |  | $ 6,727,000 |
Predecessor |  |  |  |  |  |
Related Party Transactions |  |  |  |  |  |
Capital contribution |  | $ 82,000 |  |  |  |
Members' equity | 1,125,165,000 | 986,468,000 | $ 1,024,759,000 |  |  | $ 1,067,944,000
Employee equity compensation payable |  | 31,019,000 |  |  |  |
Contingent consideration recognized | $ 26,830,000 | 58,520,000 | $ 26,224,000 |  |  |
Contingent consideration payable to related parties |  | $ 129,393,000 |  |  |  |
Predecessor | Banks | Capital contribution |  |  |  |  |  |
Related Party Transactions |  |  |  |  |  |
Capital contribution |  |  |  | $ 120,000,000 |  |
Predecessor | Employees | Capital contribution |  |  |  |  |  |
Related Party Transactions |  |  |  |  |  |
Investment by employees |  |  |  | $ 5,266,000 |  |

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