Collateralized Transactions - Schedule of Gross Amounts and Offsetting Amounts of Reverse Repurchase Agreements, Repurchase Agreements, Securities Borrowed and Securities Lending Transactions (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Reverse repurchase agreements
Gross Amounts of Recognized Assets	$ 101,749	$ 200,712
Gross Amounts Offset on the Balance Sheet	(95,011)	(200,054)
Net Amounts of Assets Presented on the Balance Sheet	6,738	658
Financial Instruments	(6,431)	0
Cash Collateral Received	0	0
Net Amount	307	658
Securities borrowed
Gross Amounts of Recognized Assets	122,238	132,368
Gross Amounts Offset on the Balance Sheet	0	0
Net Amounts of Assets Presented on the Balance Sheet	122,238	132,368
Financial Instruments	(117,374)	(128,575)
Cash Collateral Received	0	0
Net Amount	4,864	3,793
Total
Gross Amounts of Recognized Assets	223,987	333,080
Gross Amounts Offset on the Balance Sheet	(95,011)	(200,054)
Net Amounts of Assets Presented on the Balance Sheet	128,976	133,026
Financial Instruments	(123,805)	(128,575)
Cash Collateral Received	0	0
Net Amount	5,171	4,451
Repurchase agreements
Gross Amounts of Recognized Liabilities		786,532
Gross Amounts Offset on the Balance Sheet	(95,011)	(200,054)
Net Amounts of Liabilities Presented on the Balance Sheet	599,151	586,478
Financial Instruments	(597,168)	(585,289)
Cash Collateral Pledged	0	0
Net Amount	1,983	1,189
Securities loaned
Gross Amounts of Recognized Liabilities		180,270
Gross Amounts Offset on the Balance Sheet	0	0
Net Amounts of Liabilities Presented on the Balance Sheet	199,598	180,270
Financial Instruments	(188,954)	(170,176)
Cash Collateral Pledged	0	0
Net Amount	10,644	10,094
Total
Gross Amounts of Recognized Liabilities	893,760	966,802
Gross Amounts Offset on the Balance Sheet	(95,011)	(200,054)
Net Amounts of Liabilities Presented on the Balance Sheet	798,749	766,748
Financial Instruments	(786,122)	(755,465)
Cash Collateral Pledged	0	0
Net Amount	$ 12,627	$ 11,283