Investments (Contractual maturity) (Details) - USD ($) $ in Millions	Mar. 31, 2020	Dec. 31, 2019
Investments [Abstract]
Due in one year or less, cost or amortized cost	$ 1,331	$ 1,334
Due after one year through five years, cost or amortized cost	11,554	9,746
Due after five years through ten years, cost or amortized cost	13,078	14,892
Due after ten years, cost or amortized cost	12,056	12,134
Cost or Amortized Cost	38,019	38,106
Due in one year or less, estimated fair value	1,325	1,356
Due after one year through five years, estimated fair value	11,812	10,186
Due after five years through ten years, estimated fair value	13,069	15,931
Due after ten years, estimated fair value	13,877	14,714
Total Estimated Fair Value	$ 40,083	$ 42,187