Offsetting Assets and Liabilities - Offsetting Liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Securities Loaned [Abstract]
Gross Amounts of Recognized Liabilities	$ 0	$ 59,526
Gross Amounts Offset in the Statement of Financial Condition	0	0
Net Amounts of Liabilities Presented in the Statement of Financial Condition	0	59,526
Financial Instruments	0	(59,526)
Cash Collateral Pledged	0	0
Net Amount	0	0
Derivative Liability [Abstract]
Cash Collateral Pledged	(4,300)	(4,500)
Derivatives
Derivative Liability [Abstract]
Gross Amounts of Recognized Liabilities	19,259	14,822
Gross Amounts Offset in the Statement of Financial Condition	0	0
Net Amounts of Liabilities Presented in the Statement of Financial Condition	19,259	14,822
Financial Instruments	0	0
Cash Collateral Pledged	(4,276)	(4,458)
Net Amount	14,983	10,364
Exchange-traded options | Short exchange-traded options
Derivative Liability [Abstract]
Gross Amounts of Recognized Liabilities	12,348	3,782
Gross Amounts Offset in the Statement of Financial Condition	0	0
Net Amounts of Liabilities Presented in the Statement of Financial Condition	12,348	3,782
Financial Instruments	0	0
Cash Collateral Pledged	0	0
Net Amount	$ 12,348	$ 3,782