Offsetting Assets and Liabilities (Offsetting Assets) (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Derivative Asset [Abstract]
Cash Collateral Received	$ (300)	$ (500)
Securities borrowed
Securities Borrowed [Abstract]
Gross Amounts of Recognized Assets	72,916	85,371
Gross Amounts Offset in the Statement of Financial Position	0	0
Net Amounts of Assets Presented in the Statement of Financial Position	72,916	85,371
Financial Instruments	(68,096)	(82,353)
Cash Collateral Received	0	0
Net Amount	4,820	3,018
Derivatives
Derivative Asset [Abstract]
Gross Amounts of Recognized Assets	14,113	11,619
Gross Amounts Offset in the Statement of Financial Position	0	0
Net Amounts of Assets Presented in the Statement of Financial Position	14,113	11,619
Financial Instruments	0	0
Cash Collateral Received	(250)	(519)
Net Amount	13,863	11,100
Exchange-traded options | Long
Derivative Asset [Abstract]
Gross Amounts of Recognized Assets	8,233	4,981
Gross Amounts Offset in the Statement of Financial Position	0	0
Net Amounts of Assets Presented in the Statement of Financial Position	8,233	4,981
Financial Instruments	0	0
Cash Collateral Received	0	0
Net Amount	$ 8,233	$ 4,981