Offsetting Assets and Liabilities (Offsetting Assets) (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Derivative Asset [Abstract]
Cash Collateral Received	$ (2,400)	$ (500)
Securities borrowed
Securities Borrowed [Abstract]
Gross Amounts of Recognized Assets	48,302	85,371
Gross Amounts Offset in the Statement of Financial Position	0	0
Net Amounts of Assets Presented in the Statement of Financial Position	48,302	85,371
Financial Instruments	(46,858)	(82,353)
Cash Collateral Received	0	0
Net Amount	1,444	3,018
Derivatives
Derivative Asset [Abstract]
Gross Amounts of Recognized Assets	13,565	11,619
Gross Amounts Offset in the Statement of Financial Position	0	0
Net Amounts of Assets Presented in the Statement of Financial Position	13,565	11,619
Financial Instruments	0	0
Cash Collateral Received	(2,364)	(519)
Net Amount	11,201	11,100
Exchange-traded options | Long
Derivative Asset [Abstract]
Gross Amounts of Recognized Assets	5,395	4,981
Gross Amounts Offset in the Statement of Financial Position	0	0
Net Amounts of Assets Presented in the Statement of Financial Position	5,395	4,981
Financial Instruments	0	0
Cash Collateral Received	0	0
Net Amount	$ 5,395	$ 4,981