Offsetting Assets and Liabilities (Offsetting Liabilities) (Details) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Derivative Liability [Abstract]
Cash Collateral Pledged	$ (6,900)	$ (4,500)
Securities loaned
Securities Loaned [Abstract]
Gross Amounts of Recognized Liabilities	0	59,526
Gross Amounts Offset in the Statement of Financial Position	0	0
Net Amounts of Liabilities Presented in the Statement of Financial Position	0	59,526
Financial Instruments	0	(59,526)
Cash Collateral Pledged	0	0
Net Amount	0	0
Derivatives
Derivative Liability [Abstract]
Gross Amounts of Recognized Liabilities	21,329	14,822
Gross Amounts Offset in the Statement of Financial Position	0	0
Net Amounts of Liabilities Presented in the Statement of Financial Position	21,329	14,822
Financial Instruments	0	0
Cash Collateral Pledged	(6,881)	(4,458)
Net Amount	14,448	10,364
Exchange-traded options | Short
Derivative Liability [Abstract]
Gross Amounts of Recognized Liabilities	2,177	3,782
Gross Amounts Offset in the Statement of Financial Position	0	0
Net Amounts of Liabilities Presented in the Statement of Financial Position	2,177	3,782
Financial Instruments	0	0
Cash Collateral Pledged	0	0
Net Amount	$ 2,177	$ 3,782