Derivative Instruments (Offsetting Assets and Liabilities) (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Derivatives Assets
Gross Amounts of Recognized Financial Instruments	$ 4,467,949	$ 4,847,794
Gross Amounts Offset in the Statements of Financial Position	(4,467,947)	(4,804,816)
Net Amounts Presented in the Statements of Financial Position	2	42,978
Financial Instruments/Collateral	0	0
Net Amount	2	42,978
Securities Purchased under Agreements to Resell
Gross Amounts of Recognized Financial Instruments	1,003,000	675,000
Gross Amounts Offset in the Statements of Financial Position	0	0
Net Amounts Presented in the Statements of Financial Position	1,003,000	675,000
Financial Instruments/Collateral	(1,003,000)	(675,000)
Net Amount	0	0
Total Assets
Gross Amounts of Recognized Financial Instruments	5,470,949	5,522,794
Gross Amounts Offset in the Statements of Financial Position	(4,467,947)	(4,804,816)
Net Amounts Presented in the Statements of Financial Position	1,003,002	717,978
Financial Instruments/Collateral	(1,003,000)	(675,000)
Net Amount	2	42,978
Derivatives Liabilities
Gross Amounts of Recognized Financial Instruments	2,260,681	2,140,789
Gross Amounts Offset in the Statements of Financial Position	(2,095,054)	(2,134,160)
Net Amounts Presented in the Statements of Financial Position	165,627	6,629
Financial Instruments/Collateral	(66,349)	(6,629)
Net Amount	99,278	0
Securities sold under agreement to repurchase
Gross Amounts of Recognized Financial Instruments	0	0
Gross amounts Offset in the Statements of Financial Position	0	0
Net Amounts Presented in the Statements of Financial Position	0	0
Financial Instruments/Collateral	0	0
Net Amount	0	0
Total Liabilities
Gross Amounts of Recognized Financial Instruments	2,260,681	2,140,789
Gross amounts Offset in the Statements of Financial Position Agreements to Repurchase, Securities Loaned, Asset	(2,095,054)	(2,134,160)
Net Amounts Presented in the Statements of Financial Position	165,627	6,629
Financial Instruments/Collateral	(66,349)	(6,629)
Net Amount	$ 99,278	$ 0