Offsetting Assets and Liabilities (Offsetting Liabilities) (Details) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Derivative Liability [Abstract]
Cash Collateral Pledged	$ (6,400)	$ (6,200)
Securities loaned
Securities Loaned [Abstract]
Gross Amounts of Recognized Liabilities	61,719	0
Gross Amounts Offset in the Statement of Financial Position	0	0
Net Amounts of Liabilities Presented in the Statement of Financial Position	61,719	0
Financial Instruments	(61,719)	0
Cash Collateral Pledged	0	0
Net Amount	0	0
Derivatives
Derivative Liability [Abstract]
Gross Amounts of Recognized Liabilities	6,600	8,649
Gross Amounts Offset in the Statement of Financial Position	0	0
Net Amounts of Liabilities Presented in the Statement of Financial Position	6,600	8,649
Financial Instruments	0	0
Cash Collateral Pledged	(6,367)	(6,239)
Net Amount	233	2,410
Exchange-traded options | Short
Derivative Liability [Abstract]
Gross Amounts of Recognized Liabilities	13,272	692
Gross Amounts Offset in the Statement of Financial Position	0	0
Net Amounts of Liabilities Presented in the Statement of Financial Position	13,272	692
Financial Instruments	0	0
Cash Collateral Pledged	0	0
Net Amount	$ 13,272	$ 692