Investments (Contractual maturity) (Details) - USD ($) $ in Millions	Dec. 31, 2023	Dec. 31, 2022
Investments [Abstract]
Due in one year or less, cost or amortized cost	$ 1,121	$ 1,012
Due after one year through five years, cost or amortized cost	11,563	9,880
Due after five years through ten years, cost or amortized cost	13,359	13,788
Due after ten years, cost or amortized cost	16,371	16,352
Cost or Amortized Cost	42,414	41,032
Due in one year or less, estimated fair value	1,091	1,001
Due after one year through five years, estimated fair value	11,180	9,399
Due after five years through ten years, estimated fair value	12,573	12,453
Due after ten years, estimated fair value	15,581	14,774
Estimated Fair Value	$ 40,425	$ 37,627