Offsetting Assets and Liabilities (Offsetting Assets) (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Derivative Asset [Abstract]
Cash Collateral Received	$ (500)	$ (500)
Securities borrowed
Securities Borrowed [Abstract]
Gross Amounts of Recognized Assets	46,175	85,371
Gross Amounts Offset in the Statement of Financial Position	0	0
Net Amounts of Assets Presented in the Statement of Financial Position	46,175	85,371
Financial Instruments	(42,514)	(82,353)
Cash Collateral Received	0	0
Net Amount	3,661	3,018
Derivatives
Derivative Asset [Abstract]
Gross Amounts of Recognized Assets	11,397	11,619
Gross Amounts Offset in the Statement of Financial Position	0	0
Net Amounts of Assets Presented in the Statement of Financial Position	11,397	11,619
Financial Instruments	0	0
Cash Collateral Received	(530)	(519)
Net Amount	10,867	11,100
Exchange-traded options | Long
Derivative Asset [Abstract]
Gross Amounts of Recognized Assets	1,100	4,981
Gross Amounts Offset in the Statement of Financial Position	0	0
Net Amounts of Assets Presented in the Statement of Financial Position	1,100	4,981
Financial Instruments	0	0
Cash Collateral Received	0	0
Net Amount	$ 1,100	$ 4,981