Investment Information - Maturity profile of available for sale securities) (Details) - USD ($) $ in Thousands	Mar. 31, 2022	Dec. 31, 2021
Estimated Fair Value
Estimated Fair Value	$ 19,981,477	$ 19,732,825
Amortized Cost
Cost or Amortized Cost	20,650,206	19,708,561
Fixed maturities
Estimated Fair Value
Due in one year or less	313,167	300,889
Due after one year through five years	9,592,143	8,355,255
Due after five years through 10 years	4,107,319	4,689,155
Due after 10 years	476,031	505,758
Single maturity date	14,488,660	13,851,057
Estimated Fair Value	17,648,853	17,998,109
Amortized Cost
Due in one year or less	311,780	299,772
Due after one year through five years	9,884,149	8,339,387
Due after five years through 10 years	4,353,470	4,684,393
Due after 10 years	516,934	493,927
Single maturity date	15,066,333	13,817,479
Cost or Amortized Cost	18,317,693	17,973,823
Mortgage backed securities | Fixed maturities
Estimated Fair Value
Securities without single maturity date	416,138	408,477
Estimated Fair Value	416,138	408,477
Amortized Cost
Securities without single maturity date	443,648	411,110
Cost or Amortized Cost	443,648	411,110
Commercial mortgage backed securities | Fixed maturities
Estimated Fair Value
Securities without single maturity date	1,066,365	1,046,484
Estimated Fair Value	1,066,365	1,046,484
Amortized Cost
Securities without single maturity date	1,081,732	1,047,867
Cost or Amortized Cost	1,081,732	1,047,867
Asset backed securities | Fixed maturities
Estimated Fair Value
Securities without single maturity date	1,677,690	2,692,091
Estimated Fair Value	1,677,690	2,692,091
Amortized Cost
Securities without single maturity date	1,725,980	2,697,367
Cost or Amortized Cost	$ 1,725,980	$ 2,697,367