Offsetting Assets and Liabilities (Offsetting Liabilities) (Details) - USD ($) $ in Thousands	Jun. 30, 2017	Dec. 31, 2016
Derivative Liability [Abstract]
Cash Collateral Pledged	$ (5,600)	$ (6,200)
Securities loaned
Securities Loaned [Abstract]
Gross Amounts of Recognized Liabilities	28,750	0
Gross Amounts Offset in the Statement of Financial Position	0	0
Net Amounts of Liabilities Presented in the Statement of Financial Position	28,750	0
Financial Instruments	(28,616)	0
Cash Collateral Pledged	0	0
Net Amount	134	0
Derivatives
Derivative Liability [Abstract]
Gross Amounts of Recognized Liabilities	8,176	8,649
Gross Amounts Offset in the Statement of Financial Position	0	0
Net Amounts of Liabilities Presented in the Statement of Financial Position	8,176	8,649
Financial Instruments	0	0
Cash Collateral Pledged	(5,564)	(6,239)
Net Amount	2,612	2,410
Exchange-traded options | Short
Derivative Liability [Abstract]
Gross Amounts of Recognized Liabilities	7,510	692
Gross Amounts Offset in the Statement of Financial Position	0	0
Net Amounts of Liabilities Presented in the Statement of Financial Position	7,510	692
Financial Instruments	0	0
Cash Collateral Pledged	0	0
Net Amount	$ 7,510	$ 692