Collateralized Transactions - Gross Amounts Offset (Details) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Assets, Securities Borrowed
Gross Amounts Recognized	$ 1,688,222	$ 1,636,284
Gross Amounts Offset in the Statements of Financial Condition	0	0
Net Amounts Presented in the Statements of Financial Condition	1,688,222	1,636,284
Gross Amounts Not Offset in the Statement of Financial Condition, Available Collateral	1,623,281	1,575,568
Gross Amounts Not Offset in the Statement of Financial Condition, Counterparty Netting	4,581	8,277
Net Amount	60,360	52,439
Assets, Receivable from Brokers, Dealers and Clearing Organizations
Gross Amounts Recognized	21,832	65,433
Gross Amounts Offset in the Statements of Financial Condition	0	0
Net Amounts Presented in the Statements of Financial Condition	21,832	65,433
Gross Amounts Not Offset in the Statement of Financial Condition, Available Collateral	21,797	62,580
Gross Amounts Not Offset in the Statement of Financial Condition, Counterparty Netting	0	0
Net Amount	35	2,853
Total assets
Gross Amounts Recognized	1,710,054	1,701,717
Gross Amounts Offset in the Statements of Financial Condition	0	0
Net Amounts Presented in the Statements of Financial Condition	1,710,054	1,701,717
Gross Amounts Not Offset in the Statement of Financial Condition, Available Collateral	1,645,078	1,638,148
Gross Amounts Not Offset in the Statement of Financial Condition, Counterparty Netting	4,581	8,277
Net Amount	60,395	55,292
Liabilities, Securities Loaned
Gross Amounts Recognized	372,631	463,377
Gross Amounts Offset in the Statements of Financial Condition	0	0
Net Amounts Presented in the Statements of Financial Condition	372,631	463,377
Gross Amounts Not Offset in the Statement of Financial Condition, Available Collateral	358,023	440,486
Gross Amounts Not Offset in the Statement of Financial Condition, Counterparty Netting	4,581	8,277
Net Amount	10,027	14,614
Liabilities, Financial Instruments Sold Under Agreements to Repurchase
Gross Amounts Recognized	1,027,775	954,902
Gross Amounts Offset in the Statements of Financial Condition	0	0
Net Amounts Presented in the Statements of Financial Condition	1,027,775	954,902
Gross Amounts Not Offset in the Statement of Financial Condition, Available Collateral	1,027,775	954,902
Gross Amounts Not Offset in the Statement of Financial Condition, Counterparty Netting	0	0
Net Amount	0	0
Total liabilities
Gross Amounts Recognized	1,500,406	1,418,279
Gross Amounts Offset in the Statements of Financial Condition	0	0
Net Amounts Presented in the Statements of Financial Condition	1,500,406	1,418,279
Gross Amounts Not Offset in the Statement of Financial Condition, Available Collateral	1,485,798	1,395,388
Gross Amounts Not Offset in the Statement of Financial Condition, Counterparty Netting	4,581	8,277
Net Amount	10,027	$ 14,614
Other collateralized financings
Liabilities, Securities Loaned
Gross Amounts Recognized	100,000
Gross Amounts Offset in the Statements of Financial Condition	0
Net Amounts Presented in the Statements of Financial Condition	100,000
Gross Amounts Not Offset in the Statement of Financial Condition, Available Collateral	100,000
Gross Amounts Not Offset in the Statement of Financial Condition, Counterparty Netting	0
Net Amount	$ 0