Investments in Fixed Maturity Securities (Amortized cost and fair value of fixed maturity securities) (Details) - USD ($) $ in Thousands	Mar. 31, 2024	Dec. 31, 2023	Mar. 31, 2023	Dec. 31, 2022
Investments, Debt and Equity Securities [Abstract]
Amortized Cost, Due in one year or less	$ 1,980,563
Amortized Cost, Due after one year through five years	9,877,133
Amortized Cost, Due after five years through ten years	4,681,226
Amortized Cost, Due after ten years	2,640,386
Amortized Cost, Mortgaged-backed securities	2,596,170
Total investments in fixed maturity securities, Amortized Value	21,775,478
Fair Value, Due in one year or less	1,907,003
Fair Value, Due after one year through five years	9,528,047
Fair Value, Due after five years through ten years	4,577,859
Fair Value, Due after ten years	2,543,441
Fair Value, Mortgage-backed securities	2,410,862
Total investments in fixed maturity securities, Fair Value	20,967,212	$ 20,181,547
Cumulative effect adjustment resulting from changes in accounting principles	$ 38	$ 43	$ 107	$ 114