Offsetting Assets and Liabilities (Offsetting Assets) (Details) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Derivative Asset [Abstract]
Cash Collateral Received	$ (8,500)	$ (300)
Securities borrowed
Securities Borrowed [Abstract]
Gross Amounts of Recognized Assets	45,470	38,993
Gross Amounts Offset in the Statement of Financial Condition	0	0
Net Amounts of Assets Presented in the Statement of Financial Condition	45,470	38,993
Financial Instruments	(37,959)	(38,993)
Cash Collateral Received	0	0
Net Amount	7,511	0
Derivatives
Derivative Asset [Abstract]
Gross Amounts of Recognized Assets	32,566	13,491
Gross Amounts Offset in the Statement of Financial Condition	0	0
Net Amounts of Assets Presented in the Statement of Financial Condition	32,566	13,491
Financial Instruments	0	0
Cash Collateral Received	(8,479)	(251)
Net Amount	24,087	13,240
Exchange-traded options | Long
Derivative Asset [Abstract]
Gross Amounts of Recognized Assets	36,783	6,931
Gross Amounts Offset in the Statement of Financial Condition	0	0
Net Amounts of Assets Presented in the Statement of Financial Condition	36,783	6,931
Financial Instruments	0	0
Cash Collateral Received	0	0
Net Amount	$ 36,783	$ 6,931