Collateralized Transactions - Gross Amounts Offset (Details) - USD ($) $ in Thousands	Sep. 30, 2016	Dec. 31, 2015
Assets, Securities Borrowed
Gross Amounts Recognized	$ 1,958,095	$ 1,636,284
Gross Amounts Offset in the Statements of Financial Condition	0	0
Net Amounts Presented in the Statements of Financial Condition	1,958,095	1,636,284
Gross Amounts Not Offset in the Statement of Financial Condition, Available Collateral	1,909,346	1,575,568
Gross Amounts Not Offset in the Statement of Financial Condition, Counterparty Netting	7,002	8,277
Net Amount	41,747	52,439
Assets, Receivable from Brokers, Dealers and Clearing Organizations
Gross Amounts Recognized	59,980	65,433
Gross Amounts Offset in the Statements of Financial Condition	0	0
Net Amounts Presented in the Statements of Financial Condition	59,980	65,433
Gross Amounts Not Offset in the Statement of Financial Condition, Available Collateral	59,881	62,580
Gross Amounts Not Offset in the Statement of Financial Condition, Counterparty Netting	22	0
Net Amount	77	2,853
Total assets
Gross Amounts Recognized	2,018,075	1,701,717
Gross Amounts Offset in the Statements of Financial Condition	0	0
Net Amounts Presented in the Statements of Financial Condition	2,018,075	1,701,717
Gross Amounts Not Offset in the Statement of Financial Condition, Available Collateral	1,969,227	1,638,148
Gross Amounts Not Offset in the Statement of Financial Condition, Counterparty Netting	7,024	8,277
Net Amount	41,824	55,292
Liabilities, Securities Loaned
Gross Amounts Recognized	647,602	463,377
Gross Amounts Offset in the Statements of Financial Condition	0	0
Net Amounts Presented in the Statements of Financial Condition	647,602	463,377
Gross Amounts Not Offset in the Statement of Financial Condition, Available Collateral	627,544	440,486
Gross Amounts Not Offset in the Statement of Financial Condition, Counterparty Netting	7,002	8,277
Net Amount	13,056	14,614
Liabilities, Financial Instruments Sold Under Agreements to Repurchase
Gross Amounts Recognized	1,034,700	954,902
Gross Amounts Offset in the Statements of Financial Condition	0	0
Net Amounts Presented in the Statements of Financial Condition	1,034,700	954,902
Gross Amounts Not Offset in the Statement of Financial Condition, Available Collateral	1,034,678	954,902
Gross Amounts Not Offset in the Statement of Financial Condition, Counterparty Netting	22	0
Net Amount	0	0
Total liabilities
Gross Amounts Recognized	1,732,302	1,418,279
Gross Amounts Offset in the Statements of Financial Condition	0	0
Net Amounts Presented in the Statements of Financial Condition	1,732,302	1,418,279
Gross Amounts Not Offset in the Statement of Financial Condition, Available Collateral	1,712,222	1,395,388
Gross Amounts Not Offset in the Statement of Financial Condition, Counterparty Netting	7,024	8,277
Net Amount	13,056	$ 14,614
Other collateralized financings
Liabilities, Securities Loaned
Gross Amounts Recognized	50,000
Gross Amounts Offset in the Statements of Financial Condition	0
Net Amounts Presented in the Statements of Financial Condition	50,000
Gross Amounts Not Offset in the Statement of Financial Condition, Available Collateral	50,000
Gross Amounts Not Offset in the Statement of Financial Condition, Counterparty Netting	0
Net Amount	$ 0