Summary of Significant Accounting Policies (Recent accounting change) (Details) - USD ($) $ in Thousands	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Net after tax unamortized periodic pension cost	$ (7,469,237)	$ (6,768,161)
Fixed maturity securities, allowance for credit loss	36,751	37,466
Retained Earnings
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Net after tax unamortized periodic pension cost	(11,040,908)	(10,161,005)	$ (9,015,135)	$ (8,348,381)
Accumulated Other Comprehensive Income (Loss)
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Net after tax unamortized periodic pension cost	$ 586,354	$ 892,905	$ (90,900)	$ (289,714)