Investments (Contractual maturity) (Details) - USD ($) $ in Millions	Sep. 30, 2021	Dec. 31, 2020
Investments [Abstract]
Due in one year or less, cost or amortized cost	$ 1,648	$ 1,456
Due after one year through five years, cost or amortized cost	10,776	12,304
Due after five years through ten years, cost or amortized cost	13,628	12,319
Due after ten years, cost or amortized cost	14,281	12,847
Cost or Amortized Cost	40,333	38,926
Due in one year or less, estimated fair value	1,656	1,458
Due after one year through five years, estimated fair value	11,517	13,098
Due after five years through ten years, estimated fair value	14,794	13,878
Due after ten years, estimated fair value	17,093	16,170
Total Estimated Fair Value	$ 45,060	$ 44,604