Offsetting Assets and Liabilities - Offsetting Assets (Details) - USD ($) $ in Thousands	Jun. 30, 2020	Dec. 31, 2019
Derivative Asset [Abstract]
Cash Collateral Received	$ (800)	$ (300)
Securities borrowed
Securities Borrowed [Abstract]
Gross Amounts of Recognized Assets	18,109	38,993
Gross Amounts Offset in the Statement of Financial Condition	0	0
Net Amounts of Assets Presented in the Statement of Financial Condition	18,109	38,993
Financial Instruments	(17,539)	(38,993)
Cash Collateral Received	0	0
Net Amount	570	0
Derivatives
Derivative Asset [Abstract]
Gross Amounts of Recognized Assets	24,224	13,491
Gross Amounts Offset in the Statement of Financial Condition	0	0
Net Amounts of Assets Presented in the Statement of Financial Condition	24,224	13,491
Financial Instruments	0	0
Cash Collateral Received	(810)	(251)
Net Amount	23,414	13,240
Exchange-traded options | Long
Derivative Asset [Abstract]
Gross Amounts of Recognized Assets	12,151	6,931
Gross Amounts Offset in the Statement of Financial Condition	0	0
Net Amounts of Assets Presented in the Statement of Financial Condition	12,151	6,931
Financial Instruments	0	0
Cash Collateral Received	0	0
Net Amount	$ 12,151	$ 6,931