Derivative Instruments (Offsetting Assets and Liabilities) (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Derivatives Assets
Gross Amounts of Recognized Financial Instruments	$ 92,431	$ 115,086
Gross Amounts Offset in the Consolidated Statement of Financial Position	(92,431)	(115,086)
Net Amounts Presented in the Consolidated Statement of Financial Position	0	0
Financial Instruments/Collateral	0	0
Net Amount	0	0
Securities Purchased under Agreements to Resell
Gross Amounts of Recognized Financial Instruments	30,000	148,000
Gross Amounts Offset in the Consolidated Statement of Financial Position	0	0
Net Amounts Presented in the Consolidated Statement of Financial Position	30,000	148,000
Financial Instruments/Collateral	(30,000)	(148,000)
Net Amount	0	0
Total Assets
Gross Amounts of Recognized Financial Instruments	122,431	263,086
Gross Amounts Offset in the Consolidated Statement of Financial Position	(92,431)	(115,086)
Net Amounts Presented in the Consolidated Statement of Financial Position	30,000	148,000
Financial Instruments/Collateral	(30,000)	(148,000)
Net Amount	0	0
Derivatives Liabilities
Gross Amounts of Recognized Financial Instruments	55,439	74,378
Gross Amounts Offset in the Consolidated Statement of Financial Position	(54,689)	(69,718)
Net Amounts Presented in the Consolidated Statement of Financial Position	750	4,660
Financial Instruments/Collateral	0	(245)
Net Amount	750	4,415
Securities Sold under Agreements to Repurchase
Gross Amounts of Recognized Financial Instruments	0	0
Gross Amounts Offset in the Consolidated Statement of Financial Position	0	0
Net Amounts Presented in the Consolidated Statement of Financial Position	0	0
Financial Instruments/Collateral	0	0
Net Amount	0	0
Total Liabilities
Gross Amounts of Recognized Financial Instruments	55,439	74,378
Gross Amounts Offset in the Consolidated Statement of Financial Position	(54,689)	(69,718)
Net Amounts Presented in the Consolidated Statement of Financial Position	750	4,660
Financial Instruments/Collateral	0	(245)
Net Amount	$ 750	$ 4,415