Fair Value Disclosures (Narrative) (Details) (USD $)	3 Months Ended	6 Months Ended	3 Months Ended
	Jun. 30, 2013	Jun. 30, 2013	Jun. 30, 2013 Residential Mortgage-Backed Securities [Member]	Jun. 30, 2013 Commercial Mortgage-Backed Securities [Member]	Jun. 30, 2013 Corporate Debt Securities [Member]	Jun. 30, 2013 Corporate Equity Securities [Member]	Jun. 30, 2013 Collateralized Debt Obligations [Member]	Jun. 30, 2013 Loans And Other Receivables [Member]	Jul. 01, 2013 Knight Capital [Member]	Jun. 30, 2013 Knight Capital [Member]	Jul. 02, 2013 KCG Holdings [Member] Subsequent Event [Member]	Mar. 01, 2013 Jefferies [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Transfers of assets from Level 2 to Level 3	$ 54,900,000		$ 29,700,000	$ 5,600,000	$ 2,000,000	$ 2,700,000	$ 2,800,000	$ 6,900,000
Transfers of assets from Level 3 to Level 2	48,500,000		36,000,000	1,500,000	1,200,000	1,300,000	5,700,000	2,700,000
Transfers of liabilities from Level 2 to Level 3	2,300,000
Transfers of liabilities from Level 3 to Level 2	0
Net losses on Level 3 assets (realized and unrealized)	3,400,000
Net gains on Level 3 liabilities (realized and unrealized)	400,000
Excluded Securities from unobservable quantitative information		110,100,000
Cash consideration from merger, per share									$ 3.75
Cash consideration from merger									$ 192,000,000
Percentage of outstanding common stock owned that were bought out									63.00%
Percentage of outstanding common stock owned										22.00%	8.50%	28.00%