Fair Value Disclosures (Fair Value Option Election Narrative) (Details) $ in Thousands, shares in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2016 USD ($) shares	Sep. 30, 2015 USD ($)	Sep. 30, 2016 USD ($) director shares	Sep. 30, 2015 USD ($)	Dec. 31, 2015 USD ($) shares
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
90 days or more past due	$ 59,700		$ 59,700		$ 29,700
Aggregate fair value of loans and other receivables on nonaccrual status	55,100		55,100		307,500
Financial instruments owned trading assets, at fair value	15,597,243		15,597,243		$ 18,293,090
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	6,100	$ (28,300)	24,500	$ 26,700
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	$ 91,800	$ (59,200)	$ 99,700	$ (113,200)
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	$ 731,600		$ 731,600		$ 631,900
Cash consideration paid for shares	$ 475,600		$ 475,600
Number of directors appointed | director			2