Offsetting Assets and Liabilities - Offsetting Assets (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Securities Borrowed [Abstract]
Gross Amounts of Recognized Assets	$ 7,808	$ 38,993
Gross Amounts Offset in the Statement of Financial Condition	0	0
Net Amounts of Assets Presented in the Statement of Financial Condition	7,808	38,993
Financial Instruments Collateral	(7,344)	(38,993)
Cash Collateral Received	0	0
Net Amount	464	0
Derivative Asset [Abstract]
Cash Collateral Received	(400)	(300)
Derivatives
Derivative Asset [Abstract]
Gross Amounts of Recognized Assets	21,741	13,491
Gross Amounts Offset in the Statement of Financial Condition	0	0
Net Amounts of Assets Presented in the Statement of Financial Condition	21,741	13,491
Financial Instruments Collateral	0	0
Cash Collateral Received	(380)	(251)
Net Amount	21,361	13,240
Exchange-traded options | Long exchange-traded options
Derivative Asset [Abstract]
Gross Amounts of Recognized Assets	7,527	6,931
Gross Amounts Offset in the Statement of Financial Condition	0	0
Net Amounts of Assets Presented in the Statement of Financial Condition	7,527	6,931
Financial Instruments Collateral	0	0
Cash Collateral Received	0	0
Net Amount	$ 7,527	$ 6,931