Offsetting Assets and Liabilities - Offsetting Assets (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Derivative Asset [Abstract]
Cash Collateral Received	$ (4,800)	$ (500)
Securities borrowed
Securities Borrowed [Abstract]
Gross Amounts of Recognized Assets	64,856	85,371
Gross Amounts Offset in the Statement of Financial Condition	0	0
Net Amounts of Assets Presented in the Statement of Financial Condition	64,856	85,371
Financial Instruments	(64,217)	(82,353)
Cash Collateral Received	0	0
Net Amount	639	3,018
Derivatives
Derivative Asset [Abstract]
Gross Amounts of Recognized Assets	17,079	11,619
Gross Amounts Offset in the Statement of Financial Condition	0	0
Net Amounts of Assets Presented in the Statement of Financial Condition	17,079	11,619
Financial Instruments	0	0
Cash Collateral Received	(4,831)	(519)
Net Amount	12,248	11,100
Exchange-traded options | Long
Derivative Asset [Abstract]
Gross Amounts of Recognized Assets	2,568	4,981
Gross Amounts Offset in the Statement of Financial Condition	0	0
Net Amounts of Assets Presented in the Statement of Financial Condition	2,568	4,981
Financial Instruments	0	0
Cash Collateral Received	0	0
Net Amount	$ 2,568	$ 4,981