Investments (Contractual maturity) (Details) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018
Investments [Abstract]
Due in one year or less, cost or amortized cost	$ 1,334	$ 1,350
Due after one year through five years, cost or amortized cost	9,746	7,979
Due after five years through ten years, cost or amortized cost	14,892	16,859
Due after ten years, cost or amortized cost	12,134	11,893
Cost or Amortized Cost	38,106	38,081
Due in one year or less, estimated fair value	1,356	1,359
Due after one year through five years, estimated fair value	10,186	8,139
Due after five years through ten years, estimated fair value	15,931	16,870
Due after ten years, estimated fair value	14,714	13,174
Total Estimated Fair Value	$ 42,187	$ 39,542