Offsetting Assets and Liabilities - Offsetting Liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Derivative Liability [Abstract]
Cash Collateral Pledged	$ (4,500)	$ (8,800)
Securities loaned
Securities Loaned [Abstract]
Gross Amounts of Recognized Liabilities	59,526	37,960
Gross Amounts Offset in the Statement of Financial Condition	0	0
Net Amounts of Liabilities Presented in the Statement of Financial Condition	59,526	37,960
Financial Instruments	(59,526)	(37,922)
Cash Collateral Pledged	0	0
Net Amount	0	38
Derivatives
Derivative Liability [Abstract]
Gross Amounts of Recognized Liabilities	14,822	14,323
Gross Amounts Offset in the Statement of Financial Condition	0	0
Net Amounts of Liabilities Presented in the Statement of Financial Condition	14,822	14,323
Financial Instruments	0	0
Cash Collateral Pledged	(4,458)	(8,794)
Net Amount	10,364	5,529
Exchange-traded options | Short
Derivative Liability [Abstract]
Gross Amounts of Recognized Liabilities	3,782	13,585
Gross Amounts Offset in the Statement of Financial Condition	0	0
Net Amounts of Liabilities Presented in the Statement of Financial Condition	3,782	13,585
Financial Instruments	0	0
Cash Collateral Pledged	0	0
Net Amount	$ 3,782	$ 13,585