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	Sep. 30, 2017	Dec. 31, 2016
Reverse repurchase agreements
Securities Purchased under Agreements to Resell, Gross	$ 241,551	$ 24,006
Securities Purchased under Agreements to Resell, Liability	241,551	0
Securities purchased under agreements to resell	0	24,006
Securities Purchased under Agreements to Resell, Collateral, Obligation to Return Securities	0	(23,972)
Securities Purchased under Agreements to Resell, Collateral, Obligation to Return Cash	0	0
Securities Purchased under Agreements to Resell, Offset Against Collateral, Net of Not Subject to Master Netting Arrangement, Policy Election	0	34
Securities borrowed
Securities Borrowed, Gross	159,230	154,090
Securities Borrowed, Liability	0	0
Securities borrowed	159,230	154,090
Securities Borrowed, Collateral, Obligation to Return Securities	(155,587)
Securities Borrowed, Collateral, Obligation to Return Cash	0	0
Securities Borrowed, Offset Against Collateral, Net of Not Subject to Master Netting Arrangement, Policy Election	3,643	3,580
Gross Amounts of Recognized Assets	400,781	178,096
Gross Amounts Offset in the Balance Sheet	(241,551)	0
Net Amounts of Assets Presented on the Balance Sheet	159,230	178,096
Financial Instruments	(155,587)	(174,482)
Derivative Asset, Securities Purchased under Agreements to Resell, Securities Borrowed, Collateral, Obligation to Return Cash	0	0
Net Amount	3,643	3,614
Repurchase agreements
U.S. Treasury and agency securities		378,084
Securities Sold under Agreements to Repurchase, Asset	241,551	0
Net Amounts of Liabilities Presented on the Balance Sheet	398,650	378,084
Securities Sold under Agreements to Repurchase, Collateral, Right to Reclaim Securities	(392,968)	(376,273)
Securities Sold under Agreements to Repurchase, Collateral, Right to Reclaim Cash	0	0
Securities Sold under Agreements to Repurchase, Offset Against Collateral, Net of Not Subject to Master Netting Arrangement, Policy Election	5,682	1,811
Securities loaned:
Securities Loaned, Gross		179,875
Securities Loaned, Asset	0	0
Securities loaned	179,159	179,875
Securities Loaned, Collateral, Right to Reclaim Securities	(172,459)	(171,991)
Securities Loaned, Collateral, Right to Reclaim Cash	0	0
Securities Loaned, Offset Against Collateral, Net of Not Subject to Master Netting Arrangement, Policy Election	6,700	7,884
Gross amount of recognized liabilities for repurchase agreements and securities loaned	819,360	557,959
Gross Amounts Offset in the Balance Sheet	(241,551)	0
Net Amounts of Liabilities Presented on the Balance Sheet	577,809	557,959
Financial Instruments	(565,427)	(548,264)
Derivative Liability, Securities Sold under Agreements to Repurchase, Securities Loaned, Collateral, Right to Reclaim Cash	0	0
Net Amount	12,382	9,695
Securities Borrowed [Member]
Securities borrowed
Securities Borrowed, Collateral, Obligation to Return Securities		$ (150,510)
Maturity Overnight and Open [Member] | US Treasury and Government [Member]
Repurchase agreements
U.S. Treasury and agency securities	640,201
Maturity Overnight and Open [Member] | Equity Securities [Member]
Securities loaned:
Securities Loaned, Gross	$ 179,159