Intangible Assets (Roll-forward of Goodwill by Business Segment) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Goodwill [Roll Forward]
Balance at beginning of period	$ 15,541	$ 15,332
Foreign exchange revaluation and other	(270)	209
Balance at end of period	15,271	15,541
North America Commercial P&C Insurance
Goodwill [Roll Forward]
Balance at beginning of period	6,976	6,961
Foreign exchange revaluation and other	(30)	15
Balance at end of period	6,946	6,976
North American Personal P&C [Member]
Goodwill [Roll Forward]
Balance at beginning of period	2,240	2,235
Foreign exchange revaluation and other	(10)	5
Balance at end of period	2,230	2,240
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	5,004	4,817
Foreign exchange revaluation and other	(234)	187
Balance at end of period	4,770	5,004
Global Reinsurance
Goodwill [Roll Forward]
Balance at beginning of period	365	365
Foreign exchange revaluation and other	6	0
Balance at end of period	371	365
Life Insurance
Goodwill [Roll Forward]
Balance at beginning of period	822	820
Foreign exchange revaluation and other	(2)	2
Balance at end of period	$ 820	$ 822