Derivative Instruments (Offsetting Assets and Liabilities) (Details) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020
Derivatives Assets
Gross Amounts of Recognized Financial Instruments	$ 996,461	$ 14,819,236
Gross Amounts Offset in the Statements of Financial Position	(992,264)	(14,624,985)
Net Amounts Presented in the Statements of Financial Position	4,197	194,251
Financial Instruments/Collateral	0	0
Net Amount	4,197	194,251
Securities Purchased under Agreements to Resell
Gross Amounts of Recognized Financial Instruments	0	150,000
Gross Amounts Offset in the Statements of Financial Position	0	0
Net Amounts Presented in the Statements of Financial Position	0	150,000
Financial Instruments/Collateral	0	(150,000)
Net Amount	0	0
Total Assets
Gross Amounts of Recognized Financial Instruments	996,461	14,969,236
Gross Amounts Offset in the Statements of Financial Position	(992,264)	(14,624,985)
Net Amounts Presented in the Statements of Financial Position	4,197	344,251
Financial Instruments/Collateral	0	(150,000)
Net Amount	4,197	194,251
Derivatives Liabilities
Gross Amounts of Recognized Financial Instruments	1,247,310	11,952,846
Gross Amounts Offset in the Statements of Financial Position	(960,518)	(11,936,059)
Net Amounts Presented in the Statements of Financial Position	286,792	16,787
Financial Instruments/Collateral	(286,792)	(16,787)
Net Amount	0	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	1,247,310	11,952,846
Gross amounts Offset in the Statements of Financial Position Agreements to Repurchase, Securities Loaned, Asset	(960,518)	(11,936,059)
Net Amounts Presented in the Statements of Financial Position	286,792	16,787
Financial Instruments/Collateral	(286,792)	(16,787)
Net Amount	$ 0	$ 0