Offsetting Assets and Liabilities (Offsetting Liabilities) (Details) - USD ($) $ in Thousands	Mar. 31, 2017	Dec. 31, 2016
Derivative Liability [Abstract]
Cash Collateral Pledged	$ (8,100)	$ (6,200)
Securities loaned
Securities Loaned [Abstract]
Gross Amounts of Recognized Liabilities	0	0
Gross Amounts Offset in the Statement of Financial Position	0	0
Net Amounts of Liabilities Presented in the Statement of Financial Position	0	0
Financial Instruments	0	0
Cash Collateral Pledged	0	0
Net Amount	0	0
Derivatives
Derivative Liability [Abstract]
Gross Amounts of Recognized Liabilities	10,347	8,649
Gross Amounts Offset in the Statement of Financial Position	0	0
Net Amounts of Liabilities Presented in the Statement of Financial Position	10,347	8,649
Financial Instruments	0	0
Cash Collateral Pledged	(8,146)	(6,239)
Net Amount	2,201	2,410
Exchange-traded options | Short
Derivative Liability [Abstract]
Gross Amounts of Recognized Liabilities	916	692
Gross Amounts Offset in the Statement of Financial Position	0	0
Net Amounts of Liabilities Presented in the Statement of Financial Position	916	692
Financial Instruments	0	0
Cash Collateral Pledged	0	0
Net Amount	$ 916	$ 692