Significant Accounting Policies (Details) $ in Thousands	3 Months Ended	12 Months Ended
	Dec. 31, 2014 USD ($)	Dec. 31, 2015 USD ($) field	Dec. 31, 2014 USD ($)	Dec. 31, 2013 USD ($)
Long Lived Assets Held-for-sale [Line Items]
Receivables from brokers, dealers and clearing organizations	$ 2,187,500	$ 1,616,300	$ 2,187,500
Receivables from customers of securities operations	1,250,500	1,191,300	1,250,500
Provision for doubtful accounts		$ 7,353	59,695	$ 1,490
Number of oil fields | field		1
Payables to brokers, dealers and clearing organizations	2,280,100	$ 2,757,200	2,280,100
Payables to customers of securities operations	6,242,000	2,780,500	6,242,000
Real Estate [Member]
Long Lived Assets Held-for-sale [Line Items]
Carrying value of asset prior to impairment				32,300
Impaired asset fair value				12,300
Asset impairment charges				20,000
Out of period adjustment				15,400
Selling, General And Other Expenses [Member]
Long Lived Assets Held-for-sale [Line Items]
Asset impairment charges		27,700
Jefferies [Member]
Long Lived Assets Held-for-sale [Line Items]
Provision for doubtful accounts	$ 52,300
National Beef [Member]
Long Lived Assets Held-for-sale [Line Items]
Carrying value of asset prior to impairment				93,200
Impaired asset fair value				29,900
Additional costs related to closing the facility			6,900
National Beef [Member] | Selling, General And Other Expenses [Member]
Long Lived Assets Held-for-sale [Line Items]
Asset impairment charges		4,700		$ 63,300
National Beef [Member] | Compensation And Benefits [Member]
Long Lived Assets Held-for-sale [Line Items]
Additional costs related to closing the facility			$ 4,600
Juneau Energy, LLC [Member]
Long Lived Assets Held-for-sale [Line Items]
Asset impairment charges		$ 20,300