Derivative Instruments (Offsetting Assets and Liabilities) (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Derivatives Assets
Gross Amounts of Recognized Financial Instruments	$ 120,214	$ 115,086
Gross Amounts Offset in the Consolidated Statement of Financial Position	(112,098)	(115,086)
Net Amounts Presented in the Consolidated Statement of Financial Position	8,116	0
Financial Instruments/Collateral	(5,961)	0
Net Amount	2,155	0
Securities Purchased under Agreements to Resell
Gross Amounts of Recognized Financial Instruments	205,000	148,000
Gross Amounts Offset in the Consolidated Statement of Financial Position	0	0
Net Amounts Presented in the Consolidated Statement of Financial Position	205,000	148,000
Financial Instruments/Collateral	(205,000)	(148,000)
Net Amount	0	0
Total Assets
Gross Amounts of Recognized Financial Instruments	325,214	263,086
Gross Amounts Offset in the Consolidated Statement of Financial Position	(112,098)	(115,086)
Net Amounts Presented in the Consolidated Statement of Financial Position	213,116	148,000
Financial Instruments/Collateral	(210,961)	(148,000)
Net Amount	2,155	0
Derivatives Liabilities
Gross Amounts of Recognized Financial Instruments	68,355	74,378
Gross Amounts Offset in the Consolidated Statement of Financial Position	(68,355)	(69,718)
Net Amounts Presented in the Consolidated Statement of Financial Position	0	4,660
Financial Instruments/Collateral	0	(245)
Net Amount	0	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	68,355	74,378
Gross Amounts Offset in the Consolidated Statement of Financial Position	(68,355)	(69,718)
Net Amounts Presented in the Consolidated Statement of Financial Position	0	4,660
Financial Instruments/Collateral	0	(245)
Net Amount	$ 0	$ 4,415