Investments (Contractual maturity) (Details) - USD ($) $ in Millions	Mar. 31, 2024	Dec. 31, 2023
Investments [Abstract]
Due in one year or less, cost or amortized cost	$ 1,238	$ 1,121
Due after one year through five years, cost or amortized cost	11,680	11,563
Due after five years through ten years, cost or amortized cost	13,283	13,359
Due after ten years, cost or amortized cost	16,677	16,371
Cost or Amortized Cost	42,878	42,414
Due in one year or less, estimated fair value	1,207	1,091
Due after one year through five years, estimated fair value	11,308	11,180
Due after five years through ten years, estimated fair value	12,461	12,573
Due after ten years, estimated fair value	15,629	15,581
Estimated Fair Value	$ 40,605	$ 40,425