Investments (Contractual maturity) (Details) - USD ($) $ in Millions	Mar. 31, 2021	Dec. 31, 2020
Investments [Abstract]
Due in one year or less, cost or amortized cost	$ 1,468	$ 1,456
Due after one year through five years, cost or amortized cost	10,837	12,304
Due after five years through ten years, cost or amortized cost	13,640	12,319
Due after ten years, cost or amortized cost	13,312	12,847
Cost or Amortized Cost	39,257	38,926
Due in one year or less, estimated fair value	1,473	1,458
Due after one year through five years, estimated fair value	11,583	13,098
Due after five years through ten years, estimated fair value	14,685	13,878
Due after ten years, estimated fair value	15,804	16,170
Total Estimated Fair Value	$ 43,545	$ 44,604