Fair Value Disclosures (Quantitative Information About Significant Unobservable Inputs Used In Level 3 Fair Value Measurements) (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended	9 Months Ended	12 Months Ended
	Sep. 30, 2015	Sep. 30, 2015	Dec. 31, 2014	Jun. 30, 2015	Sep. 30, 2014	Jun. 30, 2014	Dec. 31, 2013
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Fair Value of Financial Instruments Owned, Derivatives and Other Contractual Agreements	$ 478,603	$ 478,603	$ 406,268
Loans [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Fair Value of Trading Liabilities	10,371	10,371	14,450	$ 10,732	$ 6,661	$ 31,534	$ 22,462
Other Secured Financings [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Fair Value of Trading Liabilities	574	574	30,825	56,060	31,666	20,288	8,711
Corporate Equity Securities [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Fair Value of Financial Instruments Owned	38,502	38,502	20,964	20,547	7,777	16,402	9,884
Corporate Debt Securities [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Fair Value of Financial Instruments Owned	24,331	24,331	22,766	31,917	36,884	31,648	25,666
Collateralized Debt Obligations [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Fair Value of Financial Instruments Owned	81,050	81,050	124,650	89,007	43,740	42,313	37,216
Residential Mortgage-Backed Securities [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Fair Value of Financial Instruments Owned	86,422	86,422	82,557	88,695	81,034	71,962	105,492
Commercial Mortgage-Backed Securities [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Fair Value of Financial Instruments Owned	15,147	15,147	26,655	17,862	19,327	24,246	17,568
Other Asset-Backed Securities [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Fair Value of Financial Instruments Owned	32,596	32,596	2,294	11,857	2,079	45,444	12,611
Loans and Finance Receivables [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Fair Value of Financial Instruments Owned	95,399	95,399	97,258	108,756	188,025	138,643	145,890
Investments at Fair Value [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Fair Value of Financial Instruments Owned	213,518	213,518	77,047	154,862	$ 102,092	$ 101,697	$ 66,931
Investment in FXCM [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Fair Value of Financial Instruments Owned	613,000	$ 613,000	0	$ 759,000
Financial Instruments Owned [Member] | Minimum [Member] | Derivatives, Liabilities [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Fair Value Inputs, Discount rate/yield		4.00%
Fair Value Inputs, Comparable loan price		$ 91
Financial Instruments Owned [Member] | Maximum [Member] | Derivatives, Liabilities [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Fair Value Inputs, Discount rate/yield		15.00%
Fair Value Inputs, Comparable loan price		$ 100
Financial Instruments Owned [Member] | Corporate Equity Securities [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Fair Value of Financial Instruments Owned	$ 17,340	$ 17,340	$ 19,814
Fair Value Inputs, Estimated Recovery Percentage			24.00%
Financial Instruments Owned [Member] | Corporate Equity Securities [Member] | Minimum [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Fair Value Inputs, Earnings before Interest, Taxes, Depreciation, and Amortization Multiple		0.046	3.4
Fair Value Inputs, Underlying stock price	$ 5	$ 5
Financial Instruments Owned [Member] | Corporate Equity Securities [Member] | Maximum [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Fair Value Inputs, Earnings before Interest, Taxes, Depreciation, and Amortization Multiple		0.085	4.7
Fair Value Inputs, Underlying stock price	7	$ 7
Financial Instruments Owned [Member] | Corporate Equity Securities [Member] | Weighted Average [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Fair Value Inputs, Earnings before Interest, Taxes, Depreciation, and Amortization Multiple		6.6	3.6
Fair Value Inputs, Underlying stock price	$ 6.0	$ 6.0
Financial Instruments Owned [Member] | Corporate Debt Securities [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Fair Value of Financial Instruments Owned	$ 24,331	$ 24,331	$ 22,766
Fair Value Inputs, Discount rate/yield		19.00%	32.00%
Fair Value Inputs, Transaction Level	$ 138.00	$ 138.00
Financial Instruments Owned [Member] | Collateralized Debt Obligations [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Fair Value of Financial Instruments Owned	$ 33,075	$ 33,075	$ 41,784
Financial Instruments Owned [Member] | Collateralized Debt Obligations [Member] | Minimum [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Fair Value Inputs, Constant prepayment rate		0.00%	0.00%
Fair Value Inputs, Constant default rate		0.00%	0.00%
Fair Value Inputs, Loss severity		25.00%	0.00%
Fair Value Inputs, Yield		10.00%	2.00%
Financial Instruments Owned [Member] | Collateralized Debt Obligations [Member] | Maximum [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Fair Value Inputs, Constant prepayment rate		20.00%	20.00%
Fair Value Inputs, Constant default rate		2.00%	2.00%
Fair Value Inputs, Loss severity		100.00%	70.00%
Fair Value Inputs, Yield		13.00%	51.00%
Financial Instruments Owned [Member] | Collateralized Debt Obligations [Member] | Weighted Average [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Fair Value Inputs, Constant prepayment rate		17.00%	13.00%
Fair Value Inputs, Constant default rate		2.00%	2.00%
Fair Value Inputs, Loss severity		40.00%	39.00%
Fair Value Inputs, Yield		11.00%	16.00%
Financial Instruments Owned [Member] | Residential Mortgage-Backed Securities [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Fair Value of Financial Instruments Owned	86,422	$ 86,422	$ 82,557
Financial Instruments Owned [Member] | Residential Mortgage-Backed Securities [Member] | Minimum [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Fair Value Inputs, Constant prepayment rate		0.00%	1.00%
Fair Value Inputs, Constant default rate		1.00%	1.00%
Fair Value Inputs, Loss severity		25.00%	20.00%
Fair Value Inputs, Yield		1.00%	3.00%
Financial Instruments Owned [Member] | Residential Mortgage-Backed Securities [Member] | Maximum [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Fair Value Inputs, Constant prepayment rate		50.00%	50.00%
Fair Value Inputs, Constant default rate		8.00%	100.00%
Fair Value Inputs, Loss severity		80.00%	80.00%
Fair Value Inputs, Yield		11.00%	13.00%
Financial Instruments Owned [Member] | Residential Mortgage-Backed Securities [Member] | Weighted Average [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Fair Value Inputs, Constant prepayment rate		12.00%	13.00%
Fair Value Inputs, Constant default rate		5.00%	14.00%
Fair Value Inputs, Loss severity		52.00%	50.00%
Fair Value Inputs, Yield		5.00%	7.00%
Financial Instruments Owned [Member] | Commercial Mortgage-Backed Securities [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Fair Value of Financial Instruments Owned	15,147	$ 15,147	$ 26,655
Fair Value Inputs, Estimated Recovery Percentage			90.00%
Financial Instruments Owned [Member] | Commercial Mortgage-Backed Securities [Member] | Minimum [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Fair Value Inputs, Yield		8.00%	8.00%
Fair Value Inputs, Cumulative loss rate		2.00%	4.00%
Financial Instruments Owned [Member] | Commercial Mortgage-Backed Securities [Member] | Maximum [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Fair Value Inputs, Yield		20.00%	12.00%
Fair Value Inputs, Cumulative loss rate		54.00%	72.00%
Financial Instruments Owned [Member] | Commercial Mortgage-Backed Securities [Member] | Weighted Average [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Fair Value Inputs, Yield		14.00%	11.00%
Fair Value Inputs, Cumulative loss rate		15.00%	15.00%
Financial Instruments Owned [Member] | Other Asset-Backed Securities [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Fair Value of Financial Instruments Owned	$ 32,596	$ 32,596	$ 2,294
Fair Value Inputs, Constant prepayment rate			8.00%
Fair Value Inputs, Constant default rate			3.00%
Fair Value Inputs, Loss severity			70.00%
Fair Value Inputs, Yield			7.00%
Financial Instruments Owned [Member] | Other Asset-Backed Securities [Member] | Minimum [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Fair Value Inputs, Constant prepayment rate		0.00%
Fair Value Inputs, Constant default rate		0.00%
Fair Value Inputs, Loss severity		0.00%
Fair Value Inputs, Yield		1.00%
Fair Value Inputs, Over Collateralization Percentage	111.00%	111.00%
Financial Instruments Owned [Member] | Other Asset-Backed Securities [Member] | Maximum [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Fair Value Inputs, Constant prepayment rate		24.00%
Fair Value Inputs, Constant default rate		9.00%
Fair Value Inputs, Loss severity		100.00%
Fair Value Inputs, Yield		25.00%
Fair Value Inputs, Over Collateralization Percentage	125.00%	125.00%
Financial Instruments Owned [Member] | Other Asset-Backed Securities [Member] | Weighted Average [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Fair Value Inputs, Constant prepayment rate		10.00%
Fair Value Inputs, Constant default rate		5.00%
Fair Value Inputs, Loss severity		77.00%
Fair Value Inputs, Yield		10.00%
Fair Value Inputs, Over Collateralization Percentage	123.00%	123.00%
Financial Instruments Owned [Member] | Loans and Finance Receivables [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Fair Value of Financial Instruments Owned	$ 85,445	$ 85,445	$ 88,154
Fair Value Inputs, Earnings before Interest, Taxes, Depreciation, and Amortization Multiple		6.9
Financial Instruments Owned [Member] | Loans and Finance Receivables [Member] | Minimum [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Fair Value Inputs, Earnings before Interest, Taxes, Depreciation, and Amortization Multiple			3.4
Fair Value Inputs, Estimated Recovery Percentage		34.00%	10.00%
Fair Value Inputs, Discount rate/yield		2.00%
Fair Value Inputs, Yield			3.00%
Fair Value Inputs, Comparable loan price		$ 91	$ 100
Financial Instruments Owned [Member] | Loans and Finance Receivables [Member] | Maximum [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Fair Value Inputs, Earnings before Interest, Taxes, Depreciation, and Amortization Multiple			8.2
Fair Value Inputs, Estimated Recovery Percentage		77.00%	41.00%
Fair Value Inputs, Discount rate/yield		13.00%
Fair Value Inputs, Yield			5.00%
Fair Value Inputs, Comparable loan price		$ 100	$ 101
Financial Instruments Owned [Member] | Loans and Finance Receivables [Member] | Weighted Average [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Fair Value Inputs, Earnings before Interest, Taxes, Depreciation, and Amortization Multiple			7.6
Fair Value Inputs, Estimated Recovery Percentage	39.00%		36.00%
Fair Value Inputs, Discount rate/yield		10.00%
Fair Value Inputs, Yield			4.00%
Fair Value Inputs, Comparable loan price		$ 97.0	$ 100.3
Financial Instruments Owned [Member] | Derivatives, Assets [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Fair Value of Financial Instruments Owned	$ 20,308	$ 20,308	$ 54,190
Fair Value Inputs, Comparable loan price			$ 100
Fair Value Inputs, Volatility		11.00%
Fair Value Inputs, Credit Spread		5.00%
Fair Value Inputs, Discount rate			17.00%
Financial Instruments Owned [Member] | Derivatives, Assets [Member] | Minimum [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Fair Value Inputs, Discount rate/yield		12.00%
Fair Value Inputs, Comparable loan price		$ 91
Fair Value Inputs, Volatility			13.00%
Financial Instruments Owned [Member] | Derivatives, Assets [Member] | Maximum [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Fair Value Inputs, Discount rate/yield		15.00%
Fair Value Inputs, Comparable loan price		$ 100
Fair Value Inputs, Volatility			23.00%
Financial Instruments Owned [Member] | Derivatives, Assets [Member] | Weighted Average [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Fair Value Inputs, Discount rate/yield		13.00%
Fair Value Inputs, Comparable loan price		$ 100.0
Fair Value Inputs, Volatility		0.00%	17.00%
Financial Instruments Owned [Member] | Investments at Fair Value [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Fair Value of Financial Instruments Owned	$ 61,379	$ 61,379	$ 32,323
Fair Value Inputs, Transaction Level			$ 50
Financial Instruments Owned [Member] | Investments at Fair Value [Member] | Minimum [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Fair Value Inputs, Transaction Level	$ 3	$ 3
Fair Value Inputs, Discount rate		15.00%	15.00%
Financial Instruments Owned [Member] | Investments at Fair Value [Member] | Maximum [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Fair Value Inputs, Transaction Level	56	$ 56
Fair Value Inputs, Discount rate		30.00%	30.00%
Financial Instruments Owned [Member] | Investments at Fair Value [Member] | Weighted Average [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Fair Value Inputs, Transaction Level	$ 10.0	$ 10.0
Fair Value Inputs, Discount rate		23.00%	23.00%
Financial Instruments Owned [Member] | Investment in FXCM [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Fair Value of Financial Instruments Owned	$ 613,000	$ 613,000
Fair Value of Senior Secured Term Loan Receivable	208,000	208,000
Fair Value of Financial Instruments Owned, Derivatives and Other Contractual Agreements	405,000	$ 405,000
Fair Value Inputs, Volatility		65.00%
Financial Instruments Owned [Member] | Investment in FXCM [Member] | Minimum [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Fair Value Inputs, Term based on the pay off		0 months
Financial Instruments Owned [Member] | Investment in FXCM [Member] | Maximum [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Fair Value Inputs, Term based on the pay off		1 year 2 months
Financial Instruments Owned [Member] | Investment in FXCM [Member] | Weighted Average [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Fair Value Inputs, Term based on the pay off		6 months
Securities Sold, Not yet Purchased [Member] | Derivatives, Liabilities [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Fair Value of Trading Liabilities	41,508	$ 41,508	$ 49,552
Fair Value Inputs, Constant prepayment rate		20.00%
Fair Value Inputs, Constant default rate		2.00%
Fair Value Inputs, Loss severity		30.00%
Fair Value Inputs, Yield		11.00%	5.00%
Fair Value Inputs, Credit Spread			45.00%
Securities Sold, Not yet Purchased [Member] | Loans [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Fair Value of Trading Liabilities	$ 10,371	$ 10,371	$ 14,450
Fair Value Inputs, Comparable loan price		$ 100	$ 100
Securities Sold, Not yet Purchased [Member] | Other Secured Financings [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Fair Value of Trading Liabilities			$ 30,825
Securities Sold, Not yet Purchased [Member] | Minimum [Member] | Derivatives, Liabilities [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Fair Value Inputs, Comparable loan price			$ 89
Fair Value Inputs, Volatility			13.00%
Securities Sold, Not yet Purchased [Member] | Minimum [Member] | Other Secured Financings [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Fair Value Inputs, Comparable loan price			$ 81
Securities Sold, Not yet Purchased [Member] | Maximum [Member] | Derivatives, Liabilities [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Fair Value Inputs, Comparable loan price			$ 100
Fair Value Inputs, Volatility			23.00%
Securities Sold, Not yet Purchased [Member] | Maximum [Member] | Other Secured Financings [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Fair Value Inputs, Comparable loan price			$ 100
Securities Sold, Not yet Purchased [Member] | Weighted Average [Member] | Derivatives, Liabilities [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Fair Value Inputs, Discount rate/yield		13.00%
Fair Value Inputs, Comparable loan price		$ 95.8	$ 92.0
Fair Value Inputs, Volatility			17.00%
Securities Sold, Not yet Purchased [Member] | Weighted Average [Member] | Other Secured Financings [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Fair Value Inputs, Comparable loan price			$ 98.7
Convertible Bond Model [Member] | Financial Instruments Owned [Member] | Corporate Debt Securities [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Fair Value Inputs, Discount rate/yield		74.00%
Foreign Exchange Option [Member] | Securities Sold, Not yet Purchased [Member] | Derivatives, Liabilities [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Fair Value Inputs, Volatility		11.00%
Equity Option [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Fair Value Inputs, Probability of Default		0.00%
Equity Option [Member] | Securities Sold, Not yet Purchased [Member] | Derivatives, Liabilities [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Fair Value Inputs, Volatility		42.00%