Derivative Instruments (Offsetting Assets and Liabilities) (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Derivatives Assets
Gross Amounts of Recognized Financial Instruments	$ 131,365	$ 77,887
Gross Amounts Offset in the Consolidated Statement of Financial Position	(115,900)	(27,078)
Net Amounts Presented in the Consolidated Statement of Financial Position	15,465	50,809
Financial Instruments/Collateral	(12,809)	(7,307)
Net Amount	2,656	43,502
Securities Purchased under Agreements to Resell
Gross Amounts of Recognized Financial Instruments	0	143,000
Gross Amounts Offset in the Consolidated Statement of Financial Position	0	0
Net Amounts Presented in the Consolidated Statement of Financial Position	0	143,000
Financial Instruments/Collateral	0	(143,000)
Net Amount	0	0
Total Assets
Gross Amounts of Recognized Financial Instruments	131,365	220,887
Gross Amounts Offset in the Consolidated Statement of Financial Position	(115,900)	(27,078)
Net Amounts Presented in the Consolidated Statement of Financial Position	15,465	193,809
Financial Instruments/Collateral	(12,809)	(150,307)
Net Amount	2,656	43,502
Derivatives Liabilities
Gross Amounts of Recognized Financial Instruments	39,763	26,862
Gross Amounts Offset in the Consolidated Statement of Financial Position	(39,763)	(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	39,763	26,862
Gross Amounts Offset in the Consolidated Statement of Financial Position	(39,763)	(26,862)
Net Amounts Presented in the Consolidated Statement of Financial Position	0	0
Financial Instruments/Collateral	0	0
Net Amount	$ 0	$ 0