Investments in Fixed Maturity Securities (Amortized cost and fair value of fixed maturity securities) (Details) - USD ($) $ in Thousands	Sep. 30, 2024	Jun. 30, 2024	Dec. 31, 2023	Sep. 30, 2023	Jun. 30, 2023	Dec. 31, 2022
Investments, Debt and Equity Securities [Abstract]
Amortized Cost, Due in one year or less	$ 1,910,941
Amortized Cost, Due after one year through five years	9,907,127
Amortized Cost, Due after five years through ten years	4,282,838
Amortized Cost, Due after ten years	3,562,724
Amortized Cost, Mortgaged-backed securities	3,339,488
Total investments in fixed maturity securities, Amortized Value	23,003,118
Fair Value, Due in one year or less	1,886,626
Fair Value, Due after one year through five years	9,724,883
Fair Value, Due after five years through ten years	4,283,690
Fair Value, Due after ten years	3,530,253
Fair Value, Mortgage-backed securities	3,240,479
Total investments in fixed maturity securities, Fair Value	22,665,931		$ 20,181,547
Cumulative effect adjustment resulting from changes in accounting principles	$ 30	$ 34	$ 43	$ 48	$ 53	$ 114