Derivative Instruments (Offsetting Assets and Liabilities) (Details) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019
Derivatives Assets
Gross Amounts of Recognized Financial Instruments	$ 16,967,913	$ 7,324,051
Gross Amounts Offset in the Statements of Financial Position	(16,963,144)	(7,324,049)
Net Amounts Presented in the Statements of Financial Position	4,769	2
Financial Instruments/Collateral	0	0
Net Amount	4,769	2
Securities Purchased under Agreements to Resell
Gross Amounts of Recognized Financial Instruments	0	302,000
Gross Amounts Offset in the Statements of Financial Position	0	0
Net Amounts Presented in the Statements of Financial Position	0	302,000
Financial Instruments/Collateral	0	(302,000)
Net Amount	0	0
Total Assets
Gross Amounts of Recognized Financial Instruments	16,967,913	7,626,051
Gross Amounts Offset in the Statements of Financial Position	(16,963,144)	(7,324,049)
Net Amounts Presented in the Statements of Financial Position	4,769	302,002
Financial Instruments/Collateral	0	(302,000)
Net Amount	4,769	2
Derivatives Liabilities
Gross Amounts of Recognized Financial Instruments	7,211,458	3,206,949
Gross Amounts Offset in the Statements of Financial Position	(6,835,378)	(3,053,132)
Net Amounts Presented in the Statements of Financial Position	376,080	153,817
Financial Instruments/Collateral	(38,706)	(820)
Net Amount	337,374	152,997
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	7,211,458	3,206,949
Gross amounts Offset in the Statements of Financial Position Agreements to Repurchase, Securities Loaned, Asset	(6,835,378)	(3,053,132)
Net Amounts Presented in the Statements of Financial Position	376,080	153,817
Financial Instruments/Collateral	(38,706)	(820)
Net Amount	$ 337,374	$ 152,997