Offsetting Assets and Liabilities - Offsetting Assets (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Securities Borrowed [Abstract]
Gross Amounts of Recognized Assets	$ 38,993	$ 64,856
Gross Amounts Offset in the Statement of Financial Condition	0	0
Net Amounts of Assets Presented in the Statement of Financial Condition	38,993	64,856
Financial Instruments	(38,993)	(64,217)
Cash Collateral Received	0	0
Net Amount	0	639
Derivative Asset [Abstract]
Cash Collateral Received	(300)	(4,800)
Derivatives
Derivative Asset [Abstract]
Gross Amounts of Recognized Assets	13,491	17,079
Gross Amounts Offset in the Statement of Financial Condition	0	0
Net Amounts of Assets Presented in the Statement of Financial Condition	13,491	17,079
Financial Instruments	0	0
Cash Collateral Received	(251)	(4,831)
Net Amount	13,240	12,248
Exchange-traded options | Long exchange-traded options
Derivative Asset [Abstract]
Gross Amounts of Recognized Assets	6,931	2,568
Gross Amounts Offset in the Statement of Financial Condition	0	0
Net Amounts of Assets Presented in the Statement of Financial Condition	6,931	2,568
Financial Instruments	0	0
Cash Collateral Received	0	0
Net Amount	$ 6,931	$ 2,568