Fair Value Disclosures (Fair Value Option Election Narrative) (Details) $ in Thousands, shares in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2015 USD ($) shares	Sep. 30, 2014 USD ($)	Sep. 30, 2015 USD ($) director shares	Sep. 30, 2014 USD ($)	Dec. 31, 2014 USD ($) shares
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
90 days or more past due	$ 13,000		$ 13,000		$ 0
Aggregate fair value of loans and other receivables on nonaccrual status	313,900		313,900		274,600
Financial instruments owned trading assets, at fair value	20,494,447		20,494,447		$ 19,612,490
KCG Holdings [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Changes in fair value of investments reflected as principal transactions	(28,300)	$ (6,500)	26,700	$ 100
HRG [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Changes in fair value of investments reflected as principal transactions	$ (59,200)	17,500	$ (113,200)	69,600
Shares owned, number | shares	46.6		46.6		46.6
Percentage of outstanding common stock owned	23.00%		23.00%		23.00%
Financial instruments owned trading assets, at fair value	$ 546,600		$ 546,600		$ 659,900
Cash consideration paid for shares	$ 475,600		$ 475,600
Number of directors appointed | director			2
Maximum ownership percentage			27.50%
Restriction on sale, maximum percentage			4.90%
Jefferies [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Changes in fair value of investments reflected as principal transactions		0		19,900
Other Merchant Banking Businesses [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Changes in fair value of investments reflected as principal transactions		$ 17,500		$ 49,700