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		Dec. 31, 2017		Dec. 31, 2016
Reverse repurchase agreements
Gross Amounts of Recognized Assets		$ 200,712		$ 24,006
Gross Amounts Offset on the Balance Sheet		(200,054)		0
Net Amounts of Assets Presented on the Balance Sheet		658		24,006
Financial Instruments		0		(23,972)
Cash Collateral Received		0		0
Net Amount		658		34
Securities borrowed
Gross Amounts of Recognized Assets		132,368	[1]	154,090	[2]
Gross Amounts Offset on the Balance Sheet		0	[1]	0	[2]
Net Amounts of Assets Presented on the Balance Sheet		132,368	[1]	154,090	[2]
Financial Instruments		(128,575)	[1]	(150,510)	[2]
Cash Collateral Received		0	[1]	0	[2]
Net Amount		3,793	[1]	3,580	[2]
Total
Gross Amounts of Recognized Assets		333,080		178,096
Gross Amounts Offset on the Balance Sheet		(200,054)		0
Net Amounts of Assets Presented on the Balance Sheet		133,026		178,096
Financial Instruments		(128,575)		(174,482)
Cash Collateral Received		0		0
Net Amount		4,451		3,614
Repurchase agreements
Gross Amounts of Recognized Liabilities				378,084
Gross Amounts Offset on the Balance Sheet		(200,054)		0
Net Amounts of Liabilities Presented on the Balance Sheet		586,478		378,084
Financial Instruments		(585,289)		(376,273)
Cash Collateral Pledged		0		0
Net Amount		1,189		1,811
Securities loaned
Gross Amounts of Recognized Liabilities	[3]			179,875
Gross Amounts Offset on the Balance Sheet		0	[4]	0	[3]
Net Amounts of Liabilities Presented on the Balance Sheet		180,270	[4]	179,875	[3]
Financial Instruments		(170,176)	[4]	(171,991)	[3]
Cash Collateral Pledged		0	[4]	0	[3]
Net Amount		10,094	[4]	7,884	[3]
Total
Gross Amounts of Recognized Liabilities		966,802		557,959
Gross Amounts Offset on the Balance Sheet		(200,054)		0
Net Amounts of Liabilities Presented on the Balance Sheet		766,748		557,959
Financial Instruments		(755,465)		(548,264)
Cash Collateral Pledged		0		0
Net Amount		$ 11,283		$ 9,695

[1] Included in receivable from brokers, dealers and clearing organizations on the consolidated balance sheet.

[2] Included in receivable from brokers, dealers and clearing organizations on the consolidated balance sheet.

[3] Included in payable to brokers, dealers and clearing organizations on the consolidated balance sheet.

[4] Included in payable to brokers, dealers and clearing organizations on the consolidated balance sheet.