Offsetting Assets and Liabilities - Offsetting Liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Derivative Liability [Abstract]
Cash Collateral Pledged	$ (8,800)	$ (6,200)
Securities Loaned [Member]
Securities Loaned [Abstract]
Gross Amounts of Recognized Liabilities	37,960	0
Gross Amounts Offset in the Statement of Financial Condition	0	0
Net Amounts of Liabilities Presented in the Statement of Financial Condition	37,960	0
Financial Instruments	(37,922)	0
Cash Collateral Pledged	0	0
Net Amount	38	0
Derivatives [Member]
Derivative Liability [Abstract]
Gross Amounts of Recognized Liabilities	14,323	8,649
Gross Amounts Offset in the Statement of Financial Condition	0	0
Net Amounts of Liabilities Presented in the Statement of Financial Condition	14,323	8,649
Financial Instruments	0	0
Cash Collateral Pledged	(8,794)	(6,239)
Net Amount	5,529	2,410
Exchange Traded Options [Member] | Short [Member]
Derivative Liability [Abstract]
Gross Amounts of Recognized Liabilities	13,585	692
Gross Amounts Offset in the Statement of Financial Condition	0	0
Net Amounts of Liabilities Presented in the Statement of Financial Condition	13,585	692
Financial Instruments	0	0
Cash Collateral Pledged	0	0
Net Amount	$ 13,585	$ 692