Offsetting Assets and Liabilities (Offsetting Assets) (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Derivative Asset [Abstract]
Cash Collateral Received	$ (300)	$ (4,800)
Securities borrowed
Securities Borrowed [Abstract]
Gross Amounts of Recognized Assets	22,596	64,856
Gross Amounts Offset in the Statement of Financial Position	0	0
Net Amounts of Assets Presented in the Statement of Financial Position	22,596	64,856
Financial Instruments	(22,596)	(64,217)
Cash Collateral Received	0	0
Net Amount	0	639
Derivatives
Derivative Asset [Abstract]
Gross Amounts of Recognized Assets	14,710	17,079
Gross Amounts Offset in the Statement of Financial Position	0	0
Net Amounts of Assets Presented in the Statement of Financial Position	14,710	17,079
Financial Instruments	0	0
Cash Collateral Received	(251)	(4,831)
Net Amount	14,459	12,248
Exchange-traded options | Long
Derivative Asset [Abstract]
Gross Amounts of Recognized Assets	5,803	2,568
Gross Amounts Offset in the Statement of Financial Position	0	0
Net Amounts of Assets Presented in the Statement of Financial Position	5,803	2,568
Financial Instruments	0	0
Cash Collateral Received	0	0
Net Amount	$ 5,803	$ 2,568