Investments in Fixed Maturity Securities (Amortized cost and fair value of fixed maturity securities) (Details) - USD ($) $ in Thousands	Mar. 31, 2025	Dec. 31, 2024	Mar. 31, 2024	Dec. 31, 2023
Investments, Debt and Equity Securities [Abstract]
Amortized Cost, Due in one year or less	$ 1,690,446
Amortized Cost, Due after one year through five years	9,241,233
Amortized Cost, Due after five years through ten years	3,964,722
Amortized Cost, Due after ten years	4,927,976
Amortized Cost, Mortgaged-backed securities	4,225,688
Total investments in fixed maturity securities, Amortized Value	24,050,065
Fair Value, Due in one year or less	1,670,948
Fair Value, Due after one year through five years	9,022,933
Fair Value, Due after five years through ten years	3,943,627
Fair Value, Due after ten years	4,887,101
Fair Value, Mortgage-backed securities	4,097,493
Total investments in fixed maturity securities, Fair Value	23,622,102	$ 22,399,426
Cumulative effect adjustment resulting from changes in accounting principles	$ 20	$ 25	$ 38	$ 43