Significant Accounting Policies Revenue Recognition (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Revenue from External Customer [Line Items]
Commissions	$ 63.0	$ 66.0	$ 69.3
Principal transactions, net	22.5	27.1	36.1
Equity research fees	$ 5.7	$ 6.5	$ 6.8
Hedge Funds [Member] | Maximum
Revenue from External Customer [Line Items]
Management Fees, Percent Fee	2.00%
Alternative Solutions [Member] | Maximum
Revenue from External Customer [Line Items]
Management Fees, Percent Fee	2.00%
Ramius Trading Strategies Managed Futures Fund [Member] | Maximum
Revenue from External Customer [Line Items]
Management Fees, Percent Fee	1.85%
Ramius Trading Strategies Managed Futures Fund [Member] | Minimum
Revenue from External Customer [Line Items]
Management Fees, Percent Fee	1.60%
Real Estate Funds [Member] | Maximum
Revenue from External Customer [Line Items]
Management Fees, Percent Fee	1.50%
Real Estate Funds [Member] | Minimum
Revenue from External Customer [Line Items]
Management Fees, Percent Fee	1.00%
Cash management [Member] | Maximum
Revenue from External Customer [Line Items]
Management Fees, Percent Fee	0.20%
During investment period [Member] | Healthcare Royalty Partners [Member]
Revenue from External Customer [Line Items]
Management Fees, Percent Fee	2.00%
After investment period [Member] | Healthcare Royalty Partners [Member]
Revenue from External Customer [Line Items]
Management Fees, Percent Fee	2.00%
Commodity trading advisory [Member] | Ramius Trading Strategies Managed Futures Fund [Member] | Maximum
Revenue from External Customer [Line Items]
Management Fees, Percent Fee	3.00%
Platform Fees, Percent Fee	1.50%
Levered vehicle [Member] | Ramius Trading Strategies Managed Futures Fund [Member] | Maximum
Revenue from External Customer [Line Items]
Management Fees, Percent Fee	1.00%
Platform Fees, Percent Fee	0.50%
Hedge Funds [Member]
Revenue from External Customer [Line Items]
Incentive Fees, Percent Fee	20.00%
Alternative Solutions [Member]
Revenue from External Customer [Line Items]
Incentive Fees, Percent Fee	10.00%