Offsetting Assets and Liabilities (Offsetting Liabilities) (Details) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Derivative Liability [Abstract]
Cash Collateral Pledged	$ (9,600)	$ (4,300)
Derivatives
Derivative Liability [Abstract]
Gross Amounts of Recognized Liabilities	40,056	19,259
Gross Amounts Offset in the Statement of Financial Condition	0	0
Net Amounts of Liabilities Presented in the Statement of Financial Condition	40,056	19,259
Financial Instruments	0	0
Cash Collateral Pledged	(9,644)	(4,276)
Net Amount	30,412	14,983
Exchange-traded options | Short
Derivative Liability [Abstract]
Gross Amounts of Recognized Liabilities	15,884	12,348
Gross Amounts Offset in the Statement of Financial Condition	0	0
Net Amounts of Liabilities Presented in the Statement of Financial Condition	15,884	12,348
Financial Instruments	0	0
Cash Collateral Pledged	0	0
Net Amount	$ 15,884	$ 12,348