Derivative Instruments (Offsetting Assets and Liabilities) (Details) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Derivatives Assets
Gross Amounts of Recognized Financial Instruments	$ 182,336	$ 77,887
Gross Amounts Offset in the Consolidated Statement of Financial Position	(163,984)	(27,078)
Net Amounts Presented in the Consolidated Statement of Financial Position	18,352	50,809
Financial Instruments/Collateral	(18,352)	(7,307)
Net Amount	0	43,502
Securities Purchased under Agreements to Resell
Gross Amounts of Recognized Financial Instruments	16,000	143,000
Gross Amounts Offset in the Consolidated Statement of Financial Position	0	0
Net Amounts Presented in the Consolidated Statement of Financial Position	16,000	143,000
Financial Instruments/Collateral	(16,000)	(143,000)
Net Amount	0	0
Total Assets
Gross Amounts of Recognized Financial Instruments	198,336	220,887
Gross Amounts Offset in the Consolidated Statement of Financial Position	(163,984)	(27,078)
Net Amounts Presented in the Consolidated Statement of Financial Position	34,352	193,809
Financial Instruments/Collateral	(34,352)	(150,307)
Net Amount	0	43,502
Derivatives Liabilities
Gross Amounts of Recognized Financial Instruments	53,828	26,862
Gross Amounts Offset in the Consolidated Statement of Financial Position	(53,828)	(26,862)
Net Amounts Presented in the Consolidated Statement of Financial Position	0	0
Financial Instruments/Collateral	0	0
Net Amount	0	0
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	53,828	26,862
Gross Amounts Offset in the Consolidated Statement of Financial Position	(53,828)	(26,862)
Net Amounts Presented in the Consolidated Statement of Financial Position	0	0
Financial Instruments/Collateral	0	0
Net Amount	$ 0	$ 0