Offsetting Assets and Liabilities - Offsetting Assets (Details) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020
Securities Borrowed [Abstract]
Gross Amounts of Recognized Assets	$ 15,384	$ 7,808
Gross Amounts Offset in the Statement of Financial Condition	0	0
Net Amounts of Assets Presented in the Statement of Financial Condition	15,384	7,808
Financial Instruments Collateral	(14,161)	(7,344)
Cash Collateral Received	0	0
Net Amount	1,223	464
Derivative Asset [Abstract]
Cash Collateral Received	(2,700)	(400)
Derivatives
Derivative Asset [Abstract]
Gross Amounts of Recognized Assets	18,159	21,741
Gross Amounts Offset in the Statement of Financial Condition	0	0
Net Amounts of Assets Presented in the Statement of Financial Condition	18,159	21,741
Financial Instruments Collateral	0	0
Cash Collateral Received	(2,712)	(380)
Net Amount	15,447	21,361
Exchange-traded options | Long
Derivative Asset [Abstract]
Gross Amounts of Recognized Assets	1,569	7,527
Gross Amounts Offset in the Statement of Financial Condition	0	0
Net Amounts of Assets Presented in the Statement of Financial Condition	1,569	7,527
Financial Instruments Collateral	0	0
Cash Collateral Received	0	0
Net Amount	$ 1,569	$ 7,527