Fair Value Disclosures (Investment in FXCM Narrative) (Details)		1 Months Ended	3 Months Ended
	Sep. 01, 2016 USD ($) director	Feb. 28, 2017 USD ($) director Employee	Mar. 31, 2017 USD ($) director	Mar. 31, 2016 USD ($)	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Jan. 31, 2015 USD ($)
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Unrealized and realized gains (losses), interest income and fees			$ 416,501,000	$ (102,483,000)
Investment in associated company			$ 2,122,479,000	1,843,470,000	$ 2,125,098,000	$ 1,757,369,000
Discount rate			10.60%
FXCM [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Senior secured term loan receivable			$ 123,000,000				$ 300,000,000
Loan receivable interest rate percentage							10.00%
Increase in Interest rate per annum each quarter							1.50%
Accrued interest rate			20.50%
Principal and interest payments received			$ 42,700,000
Unrealized and realized gains (losses), interest income and fees			$ 10,900,000	$ (53,200,000)
Expected number of positions eliminated | Employee		150
Expected number of positions eliminated, as percentage of global workforce		18.00%
Percentage of ownership owned			50.10%
Investment in associated company			$ 186,700,000
Investment in FXCM			$ 319,500,000
Global Brokerage Holdings [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Percentage of ownership owned			74.50%
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%
Investment in FXCM [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Percentage of ownership owned	49.90%		49.90%
Maximum percentage of all distributions	65.00%		65.00%
Number of directors appointed | director	6		6
Number of directors appointed by Company | director	3		3
Number of directors appointed by Co-Investee | director	3		3
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	45.00%
Distributions milestone after loan repayment	$ 350,000,000
Percentage of distributions from sale of assets or certain other events until milestone two	79.20%
Distributions milestone two after loan repayment	$ 500,000,000
Percentage of distributions from sale of assets or certain other events thereafter	51.60%
Number of directors with right to terminate management and any unvested incentive compensation | director		3
FXCM Term Loan Receivable [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Senior secured loan, extension term	1 year
National Futures Association [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Settlement amount		$ 0
Commodity Futures Trading Commission [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Settlement amount		$ 7,000,000
United States [Member] | FXCM [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Revenue, by geographical area, percentage		20.00%
Trading Securities [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
FXCM term loan			$ 132,800,000		164,500,000
Level 3 [Member] | Investment in FXCM [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Discount rate			15.00%
EBITDA multiple			5.4
Revenue multiple			1.5
Level 3 [Member] | Trading Securities [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
FXCM term loan			$ 132,800,000		$ 164,500,000