Subsequent Events	12 Months Ended
Dec. 31, 2018
Subsequent Events [Abstract]
Subsequent Events	Subsequent Events On January 2, 2019 (the "Acquisition Date"), the Company, together with its indirect wholly owned subsidiaries, Cowen International Ltd and Cowen QN Acquisition LLC, completed its previously announced acquisition (the "Acquisition") of Quarton International AG through the acquisition of all of the outstanding equity interest of Quarton International AG’s affiliated combining companies, Quarton Management AG, Quarton International Europe AG, Quarton Partners, LLC and Quarton Securities GP, LLC (which owns a U.S. Securities Exchange Commission ("SEC") registered broker-dealer that was subsequently renamed to Cowen Securities LP), comprising the U.S. and European operations of the acquired combining companies (collectively "Quarton"). Quarton is a group of leading global financial advisory companies serving the middle market. Due to the limited time since the date of the Acquisition, it is impractical for the Company to make certain business combination disclosures as of the date of this filing as the Company is still finalizing the analysis of the information necessary to provide these disclosures. As a result, the Company is unable to present the allocation of the preliminary purchase price to the fair value of assets acquired and liabilities assumed. The Company is also unable to present supplemental pro forma financial information related to the Acquisition. The Company plans to provide this information in its quarterly report on Form 10-Q for the quarter ended March 31, 2019. The acquisition will be accounted for under the acquisition method in accordance with US GAAP. As such, the results of operations for Quarton will be included in the Company's condensed consolidated statements of operations as of the Acquisition Date, and the assets acquired and liabilities assumed will be recorded at their fair value as at the Acquisition Date. Subsequent to the Acquisition, the operations of Quarton will be integrated within the Company's investment bank segment. The aggregate estimated purchase price of the Acquisition was $115.0 million. The Company expects a significant portion of the consideration to be allocated to goodwill and intangibles assets. On the Acquisition Date the Company paid an upfront consideration of $75.0 million subject to certain net working capital and other customary adjustments, with additional contingent consideration of $40.0 million that will become payable dependent on the achievement of certain milestones by Quarton in each of the first four years following the Acquisition Date, if certain conditions are met, an additional fifth year following the Acquisition Date. In addition, Quarton and the Company have established a retention bonus pool for previous Quarton employees. A portion of the preliminary purchase price was deposited into escrow, in the amount of $0.3 million, as a reserve for any future claims against the sellers of Quarton. All consideration, including a) the upfront consideration, b) amounts pursuant to any employee retention program and c) contingent consideration, consist of a combination of 80% cash and 20% shares of the Company’s Class A common stock. Shares issued on the Acquisition Date of 1,033,950 were valued based on the 30-trading day volume-weighted average price per share of $14.52 as of December 31, 2018. Any shares of Class A common stock issued in connection with any such contingent payments will be valued based on the 30-trading day volume-weighted average price per share as of the day immediately prior to the date on which such shares are to be issued. The Acquisition consideration held in escrow does not meet the definition of contingent consideration as provided under US GAAP. The amount held in escrow was included in the preliminary purchase price as representations and warranties were expected to be valid as of the Acquisition date. The Company has recognized approximately $1.9 million of acquisition-related costs, including legal, accounting, and valuation services through December 31, 2018. These costs are included in professional, advisory and other fees in the accompanying consolidated statements of operations. The Company has evaluated events that have occurred after the balance sheet date but before the financial statements are issued and has determined that there were no other subsequent events requiring adjustment or disclosure in the consolidated financial statements.

Subsequent Events

On January 2, 2019 (the "Acquisition Date"), the Company, together with its indirect wholly owned subsidiaries, Cowen International Ltd and Cowen QN Acquisition LLC, completed its previously announced acquisition (the "Acquisition") of Quarton International AG through the acquisition of all of the outstanding equity interest of Quarton International AG’s affiliated combining companies, Quarton Management AG, Quarton International Europe AG, Quarton Partners, LLC and Quarton Securities GP, LLC (which owns a U.S. Securities Exchange Commission ("SEC") registered broker-dealer that was subsequently renamed to Cowen Securities LP), comprising the U.S. and European operations of the acquired combining companies (collectively "Quarton"). Quarton is a group of leading global financial advisory companies serving the middle market.

Due to the limited time since the date of the Acquisition, it is impractical for the Company to make certain business combination disclosures as of the date of this filing as the Company is still finalizing the analysis of the information necessary to provide these disclosures. As a result, the Company is unable to present the allocation of the preliminary purchase price to the fair value of assets acquired and liabilities assumed. The Company is also unable to present supplemental pro forma financial information related to the Acquisition. The Company plans to provide this information in its quarterly report on Form 10-Q for the quarter ended March 31, 2019. The acquisition will be accounted for under the acquisition method in accordance with US GAAP. As such, the results of operations for Quarton will be included in the Company's condensed consolidated statements of operations as of the Acquisition Date, and the assets acquired and liabilities assumed will be recorded at their fair value as at the Acquisition Date. Subsequent to the Acquisition, the operations of Quarton will be integrated within the Company's investment bank segment.

The aggregate estimated purchase price of the Acquisition was $115.0 million. The Company expects a significant portion of the consideration to be allocated to goodwill and intangibles assets. On the Acquisition Date the Company paid an upfront consideration of $75.0 million subject to certain net working capital and other customary adjustments, with additional contingent consideration of $40.0 million that will become payable dependent on the achievement of certain milestones by Quarton in each of the first four years following the Acquisition Date, if certain conditions are met, an additional fifth year following the Acquisition Date. In addition, Quarton and the Company have established a retention bonus pool for previous Quarton employees. A portion of the preliminary purchase price was deposited into escrow, in the amount of $0.3 million, as a reserve for any future claims against the sellers of Quarton. All consideration, including a) the upfront consideration, b) amounts pursuant to any employee retention program and c) contingent consideration, consist of a combination of 80% cash and 20% shares of the Company’s Class A common stock. Shares issued on the Acquisition Date of 1,033,950 were valued based on the 30-trading day volume-weighted average price per share of $14.52 as of December 31, 2018. Any shares of Class A common stock issued in connection with any such contingent payments will be valued based on the 30-trading day volume-weighted average price per share as of the day immediately prior to the date on which such shares are to be issued.

The Acquisition consideration held in escrow does not meet the definition of contingent consideration as provided under US GAAP. The amount held in escrow was included in the preliminary purchase price as representations and warranties were expected to be valid as of the Acquisition date.

The Company has recognized approximately $1.9 million of acquisition-related costs, including legal, accounting, and valuation services through December 31, 2018. These costs are included in professional, advisory and other fees in the accompanying consolidated statements of operations.

The Company has evaluated events that have occurred after the balance sheet date but before the financial statements are issued and has determined that there were no other subsequent events requiring adjustment or disclosure in the consolidated financial statements.