Offsetting Assets and Liabilities (Offsetting Liabilities) (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Derivative Liability [Abstract]
Cash Collateral Pledged	$ (4,700)	$ (8,800)
Securities loaned
Securities Loaned [Abstract]
Gross Amounts of Recognized Liabilities	20,500	37,960
Gross Amounts Offset in the Statement of Financial Position	0	0
Net Amounts of Liabilities Presented in the Statement of Financial Position	20,500	37,960
Financial Instruments	(20,500)	(37,922)
Cash Collateral Pledged	0	0
Net Amount	0	38
Derivatives
Derivative Liability [Abstract]
Gross Amounts of Recognized Liabilities	13,106	14,323
Gross Amounts Offset in the Statement of Financial Position	0	0
Net Amounts of Liabilities Presented in the Statement of Financial Position	13,106	14,323
Financial Instruments	0	0
Cash Collateral Pledged	(4,724)	(8,794)
Net Amount	8,382	5,529
Exchange-traded options | Short
Derivative Liability [Abstract]
Gross Amounts of Recognized Liabilities	13,966	13,585
Gross Amounts Offset in the Statement of Financial Position	0	0
Net Amounts of Liabilities Presented in the Statement of Financial Position	13,966	13,585
Financial Instruments	0	0
Cash Collateral Pledged	0	0
Net Amount	$ 13,966	$ 13,585