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, 2018		Dec. 31, 2017
Reverse repurchase agreements
Gross Amounts of Recognized Assets		$ 82,429		$ 200,712
Gross Amounts Offset on the Balance Sheet		(82,139)		(200,054)
Securities Purchased under Agreement to Resell, Amount Not Offset Against Collateral				658
Net Amounts of Assets Presented on the Balance Sheet		290		658
Financial Instruments		0		0
Cash Collateral Received		0		0
Net Amount		290		658
Securities borrowed
Gross Amounts of Recognized Assets		108,144	[1]	132,368	[2]
Gross Amounts Offset on the Balance Sheet		0	[1]	0	[2]
Securities Borrowed, Amount Not Offset Against Collateral	[2]			132,368
Net Amounts of Assets Presented on the Balance Sheet		108,144		132,368
Financial Instruments		(105,960)	[1]	(128,575)	[2]
Cash Collateral Received		0	[1]	0	[2]
Net Amount		2,184	[1]	3,793	[2]
Total
Gross Amounts of Recognized Assets		190,573		333,080
Gross Amounts Offset on the Balance Sheet		(82,139)		(200,054)
Net Amounts of Assets Presented on the Balance Sheet		108,434		133,026
Financial Instruments		(105,960)		(128,575)
Cash Collateral Received		0		0
Net Amount		2,474		4,451
Repurchase agreements
Gross Amounts of Recognized Liabilities				786,532
Gross Amounts Offset on the Balance Sheet		(82,139)		(200,054)
Securities Sold under Agreements to Repurchase, Amount Not Offset Against Collateral				586,478
Net Amounts of Liabilities Presented on the Balance Sheet		484,218		586,478
Financial Instruments		(480,322)		(585,289)
Cash Collateral Pledged		0		0
Net Amount		3,896		1,189
Securities loaned
Gross Amounts of Recognized Liabilities	[3]			180,270
Gross Amounts Offset on the Balance Sheet		0	[4]	0	[3]
Securities Loaned, Amount Not Offset Against Collateral	[3]			180,270
Net Amounts of Liabilities Presented on the Balance Sheet		146,815		180,270
Financial Instruments		(139,232)	[4]	(170,176)	[3]
Cash Collateral Pledged		0	[4]	0	[3]
Net Amount		7,583	[4]	10,094	[3]
Total
Gross Amounts of Recognized Liabilities		713,172		966,802
Gross Amounts Offset on the Balance Sheet		(82,139)		(200,054)
Net Amounts of Liabilities Presented on the Balance Sheet		631,033		766,748
Financial Instruments		(619,554)		(755,465)
Cash Collateral Pledged		0		0
Net Amount		11,479		$ 11,283
U.S. treasury, agency and sovereign obligations | Maturity Overnight and on Demand [Member]
Repurchase agreements
Gross Amounts of Recognized Liabilities		566,357
Equity Securities [Member] | Maturity Overnight and on Demand [Member]
Securities loaned
Gross Amounts of Recognized Liabilities	[4]	$ 146,815

[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.