Investments (Contractual maturity) (Details) - USD ($) $ in Millions	Sep. 30, 2020	Dec. 31, 2019
Investments [Abstract]
Due in one year or less, cost or amortized cost	$ 1,372	$ 1,334
Due after one year through five years, cost or amortized cost	11,955	9,746
Due after five years through ten years, cost or amortized cost	13,026	14,892
Due after ten years, cost or amortized cost	12,609	12,134
Cost or Amortized Cost	38,962	38,106
Due in one year or less, estimated fair value	1,392	1,356
Due after one year through five years, estimated fair value	12,625	10,186
Due after five years through ten years, estimated fair value	14,359	15,931
Due after ten years, estimated fair value	15,518	14,714
Total Estimated Fair Value	$ 43,894	$ 42,187