Offsetting Assets and Liabilities - Offsetting Assets (Details) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019
Derivative Asset [Abstract]
Cash Collateral Received	$ (1,000)	$ (300)
Securities borrowed
Securities Borrowed [Abstract]
Gross Amounts of Recognized Assets	13,303	38,993
Gross Amounts Offset in the Statement of Financial Condition	0	0
Net Amounts of Assets Presented in the Statement of Financial Condition	13,303	38,993
Financial Instruments	(11,567)	(38,993)
Cash Collateral Received	0	0
Net Amount	1,736	0
Derivatives
Derivative Asset [Abstract]
Gross Amounts of Recognized Assets	26,670	13,491
Gross Amounts Offset in the Statement of Financial Condition	0	0
Net Amounts of Assets Presented in the Statement of Financial Condition	26,670	13,491
Financial Instruments	0	0
Cash Collateral Received	(980)	(251)
Net Amount	25,690	13,240
Exchange-traded options | Long
Derivative Asset [Abstract]
Gross Amounts of Recognized Assets	12,153	6,931
Gross Amounts Offset in the Statement of Financial Condition	0	0
Net Amounts of Assets Presented in the Statement of Financial Condition	12,153	6,931
Financial Instruments	0	0
Cash Collateral Received	0	0
Net Amount	$ 12,153	$ 6,931