Collateralized Transactions - Gross Amounts Offset (Details) - USD ($) $ in Thousands	Mar. 31, 2017	Dec. 31, 2016
Assets, Securities Borrowed
Gross Amounts Recognized	$ 1,594,442	$ 1,688,222
Gross Amounts Offset in the Statements of Financial Condition	0	0
Net Amounts Presented in the Statements of Financial Condition	1,594,442	1,688,222
Gross Amounts Not Offset in the Statement of Financial Condition, Available Collateral	1,537,957	1,623,281
Gross Amounts Not Offset in the Statement of Financial Condition, Counterparty Netting	6,225	4,581
Net Amount	50,260	60,360
Assets, Receivable from Brokers, Dealers and Clearing Organizations
Gross Amounts Recognized	13,735	21,832
Gross Amounts Offset in the Statements of Financial Condition	0	0
Net Amounts Presented in the Statements of Financial Condition	13,735	21,832
Gross Amounts Not Offset in the Statement of Financial Condition, Available Collateral	13,717	21,797
Gross Amounts Not Offset in the Statement of Financial Condition, Counterparty Netting	2	0
Net Amount	16	35
Total assets
Gross Amounts Recognized	1,608,177	1,710,054
Gross Amounts Offset in the Statements of Financial Condition	0	0
Net Amounts Presented in the Statements of Financial Condition	1,608,177	1,710,054
Gross Amounts Not Offset in the Statement of Financial Condition, Available Collateral	1,551,674	1,645,078
Gross Amounts Not Offset in the Statement of Financial Condition, Counterparty Netting	6,227	4,581
Net Amount	50,276	60,395
Liabilities, Securities Loaned
Gross Amounts Recognized	459,533	372,631
Gross Amounts Offset in the Statements of Financial Condition	0	0
Net Amounts Presented in the Statements of Financial Condition	459,533	372,631
Gross Amounts Not Offset in the Statement of Financial Condition, Available Collateral	446,050	358,023
Gross Amounts Not Offset in the Statement of Financial Condition, Counterparty Netting	6,225	4,581
Net Amount	7,258	10,027
Liabilities, Financial Instruments Sold Under Agreements to Repurchase
Gross Amounts Recognized	952,584	1,027,775
Gross Amounts Offset in the Statements of Financial Condition	0	0
Net Amounts Presented in the Statements of Financial Condition	952,584	1,027,775
Gross Amounts Not Offset in the Statement of Financial Condition, Available Collateral	952,582	1,027,775
Gross Amounts Not Offset in the Statement of Financial Condition, Counterparty Netting	2	0
Net Amount	0	0
Total liabilities
Gross Amounts Recognized	1,462,117	1,500,406
Gross Amounts Offset in the Statements of Financial Condition	0	0
Net Amounts Presented in the Statements of Financial Condition	1,462,117	1,500,406
Gross Amounts Not Offset in the Statement of Financial Condition, Available Collateral	1,448,632	1,485,798
Gross Amounts Not Offset in the Statement of Financial Condition, Counterparty Netting	6,227	4,581
Net Amount	7,258	10,027
Other collateralized financings
Liabilities, Securities Loaned
Gross Amounts Recognized	50,000	100,000
Gross Amounts Offset in the Statements of Financial Condition	0	0
Net Amounts Presented in the Statements of Financial Condition	50,000	100,000
Gross Amounts Not Offset in the Statement of Financial Condition, Available Collateral	50,000	100,000
Gross Amounts Not Offset in the Statement of Financial Condition, Counterparty Netting	0	0
Net Amount	$ 0	$ 0