Commitments and Contingencies	9 Months Ended
Commitments and Contingencies	Sep. 30, 2020
Commitments and Contingencies Disclosure [Abstract]
Commitments and Contingencies	Commitments and Contingencies In the normal course of business, the Company enters into user agreements with its dealers which provide the dealers with indemnification from third parties in the event that the electronic marketplaces of the Company infringe upon the intellectual property or other proprietary right of a third party. The Company's exposure under these user agreements is unknown as this would involve estimating future claims against the Company which have not yet occurred. However, based on its experience, the Company expects the risk of a material loss to be remote. The Company has been named as a defendant, along with other financial institutions, in antitrust class actions (consolidated into two actions) relating to trading practices in United States Treasury securities auctions. The Company has filed a motion to dismiss the actions, believes it has substantial defenses to the other plaintiff's claims and intends to defend itself vigorously. Additionally, the Company was dismissed from a class action relating to an interest rate swaps matter in 2017, but that matter continues against the remaining defendant financial institutions. The Company records its best estimate of a loss, including estimated defense costs, when the loss is considered probable and the amount of such loss can be reasonably estimated. Based on its experience, the Company believes that the amount of damages claimed in a legal proceeding is not a meaningful indicator of the potential liability. At this time, the Company cannot reasonably predict the timing or outcomes of, or estimate the amount of loss, or range of loss, if any, related to its pending legal proceedings, including the matter described above, and therefore does not have any contingency reserves established for any of these matters. Revolving Credit Facility On April 8, 2019, the Company entered into a five year, $500 million senior secured revolving credit facility (“Credit Facility”) with a syndicate of banks. The Credit Facility provides additional borrowing capacity to be used to fund ongoing working capital needs, letters of credit and for general corporate purposes, including potential future acquisitions and expansions. On November 7, 2019, TWM LLC entered into an amendment to the Revolving Credit Facility among TWM LLC and the lenders party thereto, which revised the Revolving Credit Facility to permit the pending LSEG Transaction. The amendment did not otherwise impact the terms of the Revolving Credit Facility and did not impact the amount of borrowings available to TWM LLC under the Revolving Credit Facility. Under the terms of the credit agreement that governs the Credit Facility, borrowings under the Credit Facility bear interest at a rate equal to, at the Company’s option, either (a) a base rate equal to the greatest of (i) the administrative agent’s prime rate, (ii) the federal funds effective rate plus ½ of 1.0% and (iii) one month LIBOR plus 1.0%, in each case plus 0.75%, or (b) LIBOR plus 1.75%, subject to a 0.00% floor. The credit agreement also includes a commitment fee of 0.25% for available but unborrowed amounts and other administrative fees that are payable quarterly. The Credit Facility is available until April 2024, provided the Company is in compliance with all covenants. Financial covenant requirements include maintaining minimum ratios related to interest coverage and leverage. As of September 30, 2020, there were no amounts outstanding under the Credit Facility.