Fair Value Disclosures (Investment in FXCM Narrative) (Details) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013	Jan. 31, 2015
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Unrealized and realized gains and interest income	$ 642,824	$ 662,213	$ 574,895
FXCM [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Senior secured term loan receivable	$ 192,700			$ 300,000
Term of loan receivable	2 years
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%
Balance outstanding used to calculate percentage of distributions from sale of assets or certain other events				$ 500,000
Percentage of distributions from sale of assets or certain other events thereafter				60.00%
Principal and interest payments received	$ 144,700
Unrealized and realized gains and interest income	491,300
Carrying value of Senior Secured Term Loan receivable and rights to future distributions	$ 625,700
Assumed change in price of shares (USD per share)	$ 1.00
Percentage of market price	6.00%
Reverse stock split	0.0001
Change in market value as a result of assumed change in price per share	$ 15,000
Change in assumed volatility	10.00%
Change in market value as a result of change in assumed volatility	$ 18,000
Estimated time to liquidity period	3 months
Estimated change in valuation	$ 8,000
FXCM [Member] | Maximum [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
FXCM [Member] | Minimum [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