Derivative Instruments (Offsetting Assets and Liabilities) (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Offsetting of Financial Assets, Derivatives
Gross Amounts of Recognized Financial Instruments	$ 14,819,236	$ 7,324,051
Gross Amounts Offset in the Statements of Financial Position	(14,624,985)	(7,324,049)
Net Amounts Presented in the Statements of Financial Position	194,251	2
Financial Instruments/Collateral	0	0
Net Amount	194,251	2
Securities purchased under agreement to resell
Gross Amounts of Recognized Financial Instruments	150,000	302,000
Gross Amounts Offset in the Statements of Financial Position	0	0
Net Amounts Presented in the Statements of Financial Position	150,000	302,000
Financial Instruments/Collateral	(150,000)	(302,000)
Net Amount	0	0
Total Assets
Gross Amounts of Recognized Financial Instruments	14,969,236	7,626,051
Gross Amounts Offset in the Statements of Financial Position	(14,624,985)	(7,324,049)
Net Amounts Presented in the Statements of Financial Position	344,251	302,002
Financial Instruments/Collateral	(150,000)	(302,000)
Net Amount	194,251	2
Offsetting of Financial Liabilities, Derivatives
Gross Amounts of Recognized Financial Instruments	11,952,846	3,206,949
Gross Amounts Offset in the Statements of Financial Position	(11,936,059)	(3,053,132)
Net Amounts Presented in the Statements of Financial Position	16,787	153,817
Financial Instruments/Collateral	(16,787)	(820)
Net Amount	0	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	11,952,846	3,206,949
Gross Amounts Offset in the Statements of Financial Position	(11,936,059)	(3,053,132)
Net Amounts Presented in the Statements of Financial Position	16,787	153,817
Financial Instruments/Collateral	(16,787)	(820)
Net Amount	$ 0	$ 152,997