Derivative Instruments (Offsetting Balance Sheet) (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Offsetting of Financial Assets, Derivatives
Gross Amounts of Recognized Financial Instruments	$ 115,086	$ 90,877
Gross Amounts Offset in the Consolidated Statement of Financial Position	(115,086)	(13,019)
Net Amounts Presented in the Consolidated Statement of Financial Position	0	77,858
Financial Instruments/Collateral	0	(77,858)
Net Amount	0	0
Securities purchased under agreements to resell
Gross Amounts of Recognized Financial Instruments	148,000	58,366
Gross Amounts Offset in the Consolidated Statement of Financial Position	0	0
Net Amounts Presented in the Consolidated Statement of Financial Position	148,000	58,366
Financial Instruments/Collateral	(148,000)	(58,366)
Net Amount	0	0
Total Assets
Gross Amounts of Recognized Financial Instruments	263,086	149,243
Gross Amounts Offset in the Consolidated Statement of Financial Position	(115,086)	(13,019)
Net Amounts Presented in the Consolidated Statement of Financial Position	148,000	136,224
Financial Instruments/Collateral	(148,000)	(136,224)
Net Amount	0	0
Offsetting of Financial Liabilities, Derivatives
Gross Amounts of Recognized Financial Instruments	74,378	12,854
Gross Amounts Offset in the Consolidated Statement of Financial Position	(69,718)	(12,854)
Net Amounts Presented in the Consolidated Statement of Financial Position	4,660	0
Financial Instruments/Collateral	(245)	0
Net Amount	4,415	0
Securities sold under agreements to repurchase
Gross Amounts of Recognized Financial Instruments	0	68,904
Gross Amounts Offset in the Consolidated Statement of Financial Position	0	0
Net Amounts Presented in the Consolidated Statement of Financial Position	0	68,904
Financial Instruments/Collateral	0	(68,904)
Net Amount	0	0
Total Liabilities
Gross Amounts of Recognized Financial Instruments	74,378	81,758
Gross Amounts Offset in the Consolidated Statement of Financial Position	(69,718)	(12,854)
Net Amounts Presented in the Consolidated Statement of Financial Position	4,660	68,904
Financial Instruments/Collateral	(245)	(68,904)
Net Amount	$ 4,415	$ 0