Offsetting Assets and Liabilities - Offsetting Assets (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Derivative Asset [Abstract]
Cash Collateral Received	$ (500)	$ (800)
Securities Borrowed [Member]
Securities Borrowed [Abstract]
Gross Amounts of Recognized Assets	85,371	82,814
Gross Amounts Offset in the Statement of Financial Condition	0	0
Net Amounts of Assets Presented in the Statement of Financial Condition	85,371	82,814
Financial Instruments	(82,353)	(80,277)
Cash Collateral Received	0	0
Net Amount	3,018	2,537
Derivatives [Member]
Derivative Asset [Abstract]
Gross Amounts of Recognized Assets	11,619	8,413
Gross Amounts Offset in the Statement of Financial Condition	0	0
Net Amounts of Assets Presented in the Statement of Financial Condition	11,619	8,413
Financial Instruments	0	0
Cash Collateral Received	(519)	(810)
Net Amount	11,100	7,603
Exchange Traded Options [Member] | Long [Member]
Derivative Asset [Abstract]
Gross Amounts of Recognized Assets	4,981	3,106
Gross Amounts Offset in the Statement of Financial Condition	0	0
Net Amounts of Assets Presented in the Statement of Financial Condition	4,981	3,106
Financial Instruments	0	0
Cash Collateral Received	0	0
Net Amount	$ 4,981	$ 3,106