Intangible Assets (Roll-forward of Goodwill by Business Segment) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Goodwill [Roll Forward]
Balance at beginning of period	$ 15,271	$ 15,541
Foreign exchange revaluation and other	25	(270)
Balance at end of period	15,296	15,271
North America Commercial P&C Insurance
Goodwill [Roll Forward]
Balance at beginning of period	6,946	6,976
Foreign exchange revaluation and other	9	(30)
Balance at end of period	6,955	6,946
Segment Insurance North American Personal P&C [Member] [Member]
Goodwill [Roll Forward]
Balance at beginning of period	2,230	2,240
Foreign exchange revaluation and other	4	(10)
Balance at end of period	2,234	2,230
North America Agricultural Insurance
Goodwill [Roll Forward]
Balance at beginning of period	134	134
Foreign exchange revaluation and other	0	0
Balance at end of period	134	134
Overseas General Insurance
Goodwill [Roll Forward]
Balance at beginning of period	4,770	5,004
Foreign exchange revaluation and other	15	(234)
Balance at end of period	4,785	4,770
Segment Global Reinsurance [Member]
Goodwill [Roll Forward]
Balance at beginning of period	371	365
Foreign exchange revaluation and other	0	6
Balance at end of period	371	371
Life Insurance
Goodwill [Roll Forward]
Balance at beginning of period	820	822
Foreign exchange revaluation and other	(3)	(2)
Balance at end of period	$ 817	$ 820