Income Taxes	9 Months Ended
Sep. 30, 2018
Income Tax Disclosure [Abstract]
Income Taxes	Income Taxes The taxable results of the Company’s U.S. operations are included in the consolidated income tax returns of Cowen Inc., as well as stand-alone state and local tax returns. The Company has subsidiaries that are resident in foreign countries where tax filings have to be submitted on a stand-alone basis. These subsidiaries are subject to tax in their respective countries and the Company is responsible for and, thus, reports all taxes incurred by these subsidiaries. The countries where the Company owns subsidiaries that file tax returns are United Kingdom, Luxembourg, Gibraltar, Canada and Hong Kong. The Company calculates its U.S. tax provision using the estimated annual effective tax rate methodology. The tax expense or benefit caused by an unusual or infrequent item is recorded in the quarter in which it occurs. To the extent that information is not available for the Company to fully determine the full year estimated impact of an item of income or tax adjustment, the Company calculates the tax impact of such item discretely. Accordingly, the Company uses the discrete methodology to calculate the income tax provision for its foreign subsidiaries and the tax impact of income attributable to redeemable non- controlling interests in consolidated subsidiaries and funds. Based on these methodologies, the Company’s effective income tax rate was 16.88% and 6.32% for the nine months ended September 30, 2018 and 2017, respectively. During the nine months ended September 30, 2018, the unusual or infrequent items whose tax impact were recorded discretely related primarily to state and foreign taxes, as well as stock compensation. For the nine months ended September 30, 2018 and 2017, the effective tax rate differs from the statutory rate of 21% primarily due to income attributable to redeemable non-controlling interests in consolidated subsidiaries and funds, stock compensation, state and foreign taxes, as well as other nondeductible expenses. The Company recorded an uncertain tax position liability of $4.9 million and $5.4 million as of September 30, 2018 and December 31, 2017, respectively, related to New York State and New York City tax matters. The Company records deferred tax assets and liabilities for the future tax benefit or expense that will result from differences between the carrying value of its assets for income tax purposes and for financial reporting purposes, as well as for operating or capital loss and tax credit carryovers. A valuation allowance is recorded to bring the net deferred tax assets to a level that, in management's view, is more likely than not to be realized in the foreseeable future. This level will be estimated based on a number of factors, especially the amount of net deferred tax assets of the Company that are actually expected to be realized, for tax purposes, in the foreseeable future. As of September 30, 2018 and December 31, 2017, the Company recorded a valuation allowance against deferred tax assets related to its foreign net operating losses. The Company is subject to examination by the United States Internal Revenue Service as well as state, local and foreign tax authorities in jurisdictions where the Company has significant business operations, such as New York, United Kingdom, and Luxembourg. In October, the Company concluded an IRS audit for tax year 2014 with no financial statement impact. The Company continues to permanently reinvest the capital and accumulated earnings of its United Kingdom, Canadian and Hong Kong subsidiaries.

Income Taxes

The taxable results of the Company’s U.S. operations are included in the consolidated income tax returns of Cowen Inc., as well as stand-alone state and local tax returns. The Company has subsidiaries that are resident in foreign countries where tax filings have to be submitted on a stand-alone basis. These subsidiaries are subject to tax in their respective countries and the Company is responsible for and, thus, reports all taxes incurred by these subsidiaries. The countries where the Company owns subsidiaries that file tax returns are United Kingdom, Luxembourg, Gibraltar, Canada and Hong Kong.

The Company calculates its U.S. tax provision using the estimated annual effective tax rate methodology. The tax expense or benefit caused by an unusual or infrequent item is recorded in the quarter in which it occurs. To the extent that information is not available for the Company to fully determine the full year estimated impact of an item of income or tax adjustment, the Company calculates the tax impact of such item discretely. Accordingly, the Company uses the discrete methodology to calculate the income tax provision for its foreign subsidiaries and the tax impact of income attributable to redeemable non- controlling interests in consolidated subsidiaries and funds. Based on these methodologies, the Company’s effective income tax rate was 16.88% and 6.32% for the nine months ended September 30, 2018 and 2017, respectively. During the nine months ended September 30, 2018, the unusual or infrequent items whose tax impact were recorded discretely related primarily to state and foreign taxes, as well as stock compensation.

For the nine months ended September 30, 2018 and 2017, the effective tax rate differs from the statutory rate of 21% primarily due to income attributable to redeemable non-controlling interests in consolidated subsidiaries and funds, stock compensation, state and foreign taxes, as well as other nondeductible expenses.

The Company recorded an uncertain tax position liability of $4.9 million and $5.4 million as of September 30, 2018 and December 31, 2017, respectively, related to New York State and New York City tax matters.

The Company records deferred tax assets and liabilities for the future tax benefit or expense that will result from differences between the carrying value of its assets for income tax purposes and for financial reporting purposes, as well as for operating or capital loss and tax credit carryovers. A valuation allowance is recorded to bring the net deferred tax assets to a level that, in management's view, is more likely than not to be realized in the foreseeable future. This level will be estimated based on a number of factors, especially the amount of net deferred tax assets of the Company that are actually expected to be realized, for tax purposes, in the foreseeable future. As of September 30, 2018 and December 31, 2017, the Company recorded a valuation allowance against deferred tax assets related to its foreign net operating losses.

The Company is subject to examination by the United States Internal Revenue Service as well as state, local and foreign tax authorities in jurisdictions where the Company has significant business operations, such as New York, United Kingdom, and Luxembourg. In October, the Company concluded an IRS audit for tax year 2014 with no financial statement impact.

The Company continues to permanently reinvest the capital and accumulated earnings of its United Kingdom, Canadian and Hong Kong subsidiaries.