Offsetting Assets and Liabilities - Offsetting Liabilities (Details) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020
Offsetting Liabilities [Line Items]
Cash Collateral Pledged	$ (3,400)	$ (6,400)
Derivatives
Offsetting Liabilities [Line Items]
Gross Amounts of Recognized Liabilities	15,276	30,537
Gross Amounts Offset in the Statement of Financial Condition	0	0
Net Amounts of Liabilities Presented in the Statement of Financial Condition	15,276	30,537
Financial Instruments Collateral	0	0
Cash Collateral Pledged	(3,354)	(6,374)
Net Amount	11,922	24,163
Exchange-traded options | Short
Offsetting Liabilities [Line Items]
Gross Amounts of Recognized Liabilities	701	12,486
Gross Amounts Offset in the Statement of Financial Condition	0	0
Net Amounts of Liabilities Presented in the Statement of Financial Condition	701	12,486
Financial Instruments Collateral	0	0
Cash Collateral Pledged	0	0
Net Amount	$ 701	$ 12,486