Derivative Instruments (Offsetting Assets and Liabilities) (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Offsetting of Financial Assets, Derivatives
Gross Amounts of Recognized Financial Instruments	$ 7,324,051	$ 4,847,794
Gross Amounts Offset in the Statements of Financial Position	(7,324,049)	(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 agreement to resell
Gross Amounts of Recognized Financial Instruments	302,000	675,000
Gross Amounts Offset in the Statements of Financial Position	0	0
Net Amounts Presented in the Statements of Financial Position	302,000	675,000
Financial Instruments/Collateral	(302,000)	(675,000)
Net Amount	0	0
Total Assets
Gross Amounts of Recognized Financial Instruments	7,626,051	5,522,794
Gross Amounts Offset in the Statements of Financial Position	(7,324,049)	(4,804,816)
Net Amounts Presented in the Statements of Financial Position	302,002	717,978
Financial Instruments/Collateral	(302,000)	(675,000)
Net Amount	2	42,978
Offsetting of Financial Liabilities, Derivatives
Gross Amounts of Recognized Financial Instruments	3,206,949	2,140,789
Gross Amounts Offset in the Statements of Financial Position	(3,053,132)	(2,134,160)
Net Amounts Presented in the Statements of Financial Position	153,817	6,629
Financial Instruments/Collateral	(820)	(6,629)
Net Amount	152,997	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	3,206,949	2,140,789
Gross Amounts Offset in the Statements of Financial Position	(3,053,132)	(2,134,160)
Net Amounts Presented in the Statements of Financial Position	153,817	6,629
Financial Instruments/Collateral	(820)	(6,629)
Net Amount	$ 152,997	$ 0