Offsetting Assets and Liabilities (Offsetting Liabilities) (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Derivative Liability [Abstract]
Cash Collateral Pledged	$ (3,000)	$ (8,800)
Securities loaned
Securities Loaned [Abstract]
Gross Amounts of Recognized Liabilities	85,805	37,960
Gross Amounts Offset in the Statement of Financial Position	0	0
Net Amounts of Liabilities Presented in the Statement of Financial Position	85,805	37,960
Financial Instruments	(84,274)	(37,922)
Cash Collateral Pledged	0	0
Net Amount	1,531	38
Derivatives
Derivative Liability [Abstract]
Gross Amounts of Recognized Liabilities	14,552	14,323
Gross Amounts Offset in the Statement of Financial Position	0	0
Net Amounts of Liabilities Presented in the Statement of Financial Position	14,552	14,323
Financial Instruments	0	0
Cash Collateral Pledged	(3,028)	(8,794)
Net Amount	11,524	5,529
Exchange-traded options | Short
Derivative Liability [Abstract]
Gross Amounts of Recognized Liabilities	18,342	13,585
Gross Amounts Offset in the Statement of Financial Position	0	0
Net Amounts of Liabilities Presented in the Statement of Financial Position	18,342	13,585
Financial Instruments	0	0
Cash Collateral Pledged	0	0
Net Amount	$ 18,342	$ 13,585