Segment Reporting Segment Reporting (Policies)	12 Months Ended
Dec. 31, 2012
Segment Reporting [Abstract]
Segment Reporting, Policy	The performance measure for these segments is Economic Income (Loss), which management uses to evaluate the financial performance of and make operating decisions for the segments including determining appropriate compensation levels. In general, Economic Income (Loss) is a pre-tax measure that (i) eliminates the impact of consolidation for consolidated funds, (ii) excludes equity award expense related to the November 2009 Ramius/Cowen transaction, (iii) excludes certain other acquisition-related and/or reorganization expenses (including the discontinued operations of LaBranche), (iv) excludes goodwill impairment and (v) excludes the bargain purchase gain which resulted from the LaBranche acquisition (See Note 2). In addition, Economic Income (Loss) revenues include investment income that represents the income the Company has earned in investing its own capital, including realized and unrealized gains and losses, interest and dividends, net of associated investment related expenses. For US GAAP purposes, these items are included in each of their respective line items. Economic Income (Loss) revenues also include management fees, incentive income and investment income earned through the Company's investment as a general partner in certain real estate entities and the Company's investment in the Value and Opportunity business. For US GAAP purposes, all of these items are recorded in other income (loss). In addition, Economic Income (Loss) expenses are reduced by reimbursement from affiliates, which for US GAAP purposes is presented gross as part of revenue. As further stated below, one major difference between Economic Income (Loss) and US GAAP net income (loss) is that Economic Income (Loss) presents the segments' results of operations without the impact resulting from the full consolidation of any of the Consolidated Funds. Consolidation of these funds results in including in income the pro rata share of the income or loss attributable to other owners of such entities which is reflected in net income (loss) attributable to redeemable non-controlling interest in consolidated subsidiaries in the accompanying consolidated statements of operations. This pro rata share has no effect on the overall financial performance for the alternative investment segment, as ultimately, this income or loss is not income or loss for the alternative investment segment itself. Included in Economic Income (Loss) is the actual pro rata share of the income or loss attributable to the Company as an investor in such entities, which is relevant in management making operating decisions and evaluating financial performance.

The performance measure for these segments is Economic Income (Loss), which management uses to evaluate the financial performance of and make operating decisions for the segments including determining appropriate compensation levels.

In general, Economic Income (Loss) is a pre-tax measure that (i) eliminates the impact of consolidation for consolidated funds, (ii) excludes equity award expense related to the November 2009 Ramius/Cowen transaction, (iii) excludes certain other acquisition-related and/or reorganization expenses (including the discontinued operations of LaBranche), (iv) excludes goodwill impairment and (v) excludes the bargain purchase gain which resulted from the LaBranche acquisition (See Note 2). In addition, Economic Income (Loss) revenues include investment income that represents the income the Company has earned in investing its own capital, including realized and unrealized gains and losses, interest and dividends, net of associated investment related expenses. For US GAAP purposes, these items are included in each of their respective line items. Economic Income (Loss) revenues also include management fees, incentive income and investment income earned through the Company's investment as a general partner in certain real estate entities and the Company's investment in the Value and Opportunity business. For US GAAP purposes, all of these items are recorded in other income (loss). In addition, Economic Income (Loss) expenses are reduced by reimbursement from affiliates, which for US GAAP purposes is presented gross as part of revenue.

As further stated below, one major difference between Economic Income (Loss) and US GAAP net income (loss) is that Economic Income (Loss) presents the segments' results of operations without the impact resulting from the full consolidation of any of the Consolidated Funds. Consolidation of these funds results in including in income the pro rata share of the income or loss attributable to other owners of such entities which is reflected in net income (loss) attributable to redeemable non-controlling interest in consolidated subsidiaries in the accompanying consolidated statements of operations. This pro rata share has no effect on the overall financial performance for the alternative investment segment, as ultimately, this income or loss is not income or loss for the alternative investment segment itself. Included in Economic Income (Loss) is the actual pro rata share of the income or loss attributable to the Company as an investor in such entities, which is relevant in management making operating decisions and evaluating financial performance.