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	6 Months Ended	12 Months Ended
	Jun. 30, 2015	Jun. 30, 2015	Dec. 31, 2014	Mar. 31, 2015	Jun. 30, 2014	Mar. 31, 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	$ 402,089	$ 402,089	$ 406,268
Loans [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Fair Value of Trading Liabilities	10,732	10,732	14,450	$ 9,327	$ 31,534	$ 10,260	$ 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	56,060	56,060	30,825	65,602	20,288	30,394	8,711
Equity Securities [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Fair Value of Financial Instruments Owned	20,547	20,547	20,964	18,210	16,402	12,341	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	31,917	31,917	22,766	24,795	31,648	29,315	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	89,007	89,007	124,650	96,837	42,313	66,028	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	88,695	88,695	82,557	79,953	71,962	116,992	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	17,862	17,862	26,655	24,629	24,246	17,486	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	11,857	11,857	2,294	7,146	45,444	2,375	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	108,756	108,756	97,258	111,410	138,643	128,832	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	154,862	154,862	77,047	151,365	$ 101,697	$ 102,780	$ 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	759,000	$ 759,000	0	$ 947,000
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, Yield		5.00%
Fair Value Inputs, Comparable loan price		$ 83
Fair Value Inputs, Volatility		11.00%
Financial Instruments Owned [Member] | Minimum [Member] | Securities Sold, Not yet Purchased [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Fair Value Inputs, Comparable loan price		$ 71
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, Yield		8.00%
Fair Value Inputs, Comparable loan price		$ 100
Fair Value Inputs, Volatility		23.00%
Financial Instruments Owned [Member] | Maximum [Member] | Securities Sold, Not yet Purchased [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Fair Value Inputs, Comparable loan price		$ 100
Financial Instruments Owned [Member] | Equity Securities [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Fair Value of Financial Instruments Owned	$ 18,647	$ 18,647
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			$ 19,814
Fair Value Inputs, Underlying stock price	$ 5.3	$ 5.3
Fair Value Inputs, Estimated Recovery Percentage		79.00%	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		0.048	3.4
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		0.059	4.7
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		5.4	3.6
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	$ 31,917	$ 31,917	$ 22,766
Fair Value Inputs, Discount rate/yield			32.00%
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	46,565	$ 46,565	$ 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		22.00%	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		39.00%	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	88,695	$ 88,695	$ 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		30.00%	20.00%
Fair Value Inputs, Yield		0.00%	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		100.00%	100.00%
Fair Value Inputs, Loss severity		90.00%	80.00%
Fair Value Inputs, Yield		12.00%	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		13.00%	13.00%
Fair Value Inputs, Constant default rate		17.00%	14.00%
Fair Value Inputs, Loss severity		59.00%	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	17,862	$ 17,862	$ 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		12.00%	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		21.00%	12.00%
Fair Value Inputs, Cumulative loss rate		49.00%	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		16.00%	11.00%
Fair Value Inputs, Cumulative loss rate		10.00%	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	11,857	$ 11,857	$ 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		3.00%
Fair Value Inputs, Constant default rate		3.00%
Fair Value Inputs, Loss severity		60.00%
Fair Value Inputs, Yield		5.00%
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		6.00%
Fair Value Inputs, Constant default rate		6.00%
Fair Value Inputs, Loss severity		70.00%
Fair Value Inputs, Yield		7.00%
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		3.00%
Fair Value Inputs, Constant default rate		5.00%
Fair Value Inputs, Loss severity		69.00%
Fair Value Inputs, Yield		5.00%
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	$ 99,094	$ 99,094	$ 88,154
Fair Value Inputs, Earnings before Interest, Taxes, Depreciation, and Amortization Multiple		7.3
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%
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		10.00%	10.00%
Fair Value Inputs, Yield		3.00%	3.00%
Fair Value Inputs, Comparable loan price		$ 99	$ 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		75.00%	41.00%
Fair Value Inputs, Yield		10.00%	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	36.00%		36.00%
Fair Value Inputs, Yield		8.00%	4.00%
Fair Value Inputs, Comparable loan price		$ 99.9	$ 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	$ 40,012	$ 40,012	$ 54,190
Fair Value Inputs, Yield		8.00%
Fair Value Inputs, Comparable loan price			$ 100
Fair Value Inputs, Discount rate		24.00%	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, Comparable loan price		$ 83
Fair Value Inputs, Volatility		11.00%	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, Comparable loan price		$ 100
Fair Value Inputs, Volatility		23.00%	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, Comparable loan price		$ 99.9
Fair Value Inputs, Volatility		13.00%	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	$ 30,146	$ 30,146	$ 32,323
Fair Value Inputs, Transaction Level	$ 56	$ 56	$ 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, 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, 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, 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	$ 759,000	$ 759,000
Fair Value of Senior Secured Term Loan Receivable	236,000	236,000
Fair Value of Financial Instruments Owned, Derivatives and Other Contractual Agreements	523,000	$ 523,000
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		3 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 6 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 18 days
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	38,426	$ 38,426	$ 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	10,732	$ 10,732	$ 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	$ 56,060	$ 56,060	$ 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, Yield		8.00%
Fair Value Inputs, Comparable loan price		$ 88.7	$ 92.0
Fair Value Inputs, Volatility		13.00%	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		$ 97.0	$ 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		35.00%
Discounted Cash Flows [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		22.00%