Fair Value Disclosures (Investment in FXCM Narrative) (Details) $ / shares in Units, $ in Thousands	1 Months Ended	3 Months Ended		9 Months Ended
	Jan. 31, 2015 USD ($)	Sep. 30, 2015 USD ($) $ / shares	Sep. 30, 2014 USD ($)	Sep. 30, 2015 USD ($) $ / shares	Sep. 30, 2014 USD ($)
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Unrealized and realized gains and interest income		$ (253,145)	$ 160,345	$ 548,328	$ 617,133
FXCM [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Senior secured term loan receivable	$ 300,000	202,900		202,900
Loan receivable interest rate percentage	10.00%
Increase in Interest rate per annum each quarter	1.50%
Percentage of distributions from sale of assets or certain other events until loan repayment	100.00%
Percentage of distributions from sale of assets or certain other events until next milestone amount	50.00%
Distributions milestone after loan repayment	$ 350,000
Percentage of distributions from sale of assets or certain other events until next milestone as a percentage of balance outstanding	90.00%
Multiple of loan outstanding	2
Percentage of distributions from sale of assets or certain other events thereafter	60.00%
Principal and interest payments received				127,300
Unrealized and realized gains and interest income		(113,200)		461,300
Carrying value of Senior Secured Term Loan receivable and rights to future distributions		$ 613,000		$ 613,000
Assumed change in price of shares | $ / shares		$ 1.00		$ 1.00
Percentage of market price		11.00%		11.00%
Change in market value as a result of assumed change in price per share		$ 22,000		$ 22,000
Change in assumed volatility		10.00%		10.00%
Change in market value as a result of change in assumed volatility		$ 25,000		$ 25,000
Maximum [Member] | FXCM [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Loan receivable interest rate percentage	20.50%
Balance outstanding used to calculate percentage of distributions from sale of assets or certain other events	$ 680,000
Minimum [Member] | FXCM [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Balance outstanding used to calculate percentage of distributions from sale of assets or certain other events	$ 500,000