Offsetting Assets and Liabilities (Offsetting Assets) (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Derivative Asset [Abstract]
Cash Collateral Received	$ (1,100)	$ (4,800)
Securities borrowed
Securities Borrowed [Abstract]
Gross Amounts of Recognized Assets	55,031	64,856
Gross Amounts Offset in the Statement of Financial Position	0	0
Net Amounts of Assets Presented in the Statement of Financial Position	55,031	64,856
Financial Instruments	(54,249)	(64,217)
Cash Collateral Received	0	0
Net Amount	782	639
Derivatives
Derivative Asset [Abstract]
Gross Amounts of Recognized Assets	10,054	17,079
Gross Amounts Offset in the Statement of Financial Position	0	0
Net Amounts of Assets Presented in the Statement of Financial Position	10,054	17,079
Financial Instruments	0	0
Cash Collateral Received	(1,141)	(4,831)
Net Amount	8,913	12,248
Exchange-traded options | Long
Derivative Asset [Abstract]
Gross Amounts of Recognized Assets	3,046	2,568
Gross Amounts Offset in the Statement of Financial Position	0	0
Net Amounts of Assets Presented in the Statement of Financial Position	3,046	2,568
Financial Instruments	0	0
Cash Collateral Received	0	0
Net Amount	$ 3,046	$ 2,568