Intangible Assets (Roll-forward of Goodwill by Business Segment) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Goodwill [Roll Forward]
Balance at beginning of period	$ 15,332	$ 4,796
Foreign exchange revaluation and other	209	22
Balance at end of period	15,541	15,332
North America Commercial P&C Insurance
Goodwill [Roll Forward]
Balance at beginning of period	6,961	1,203
Foreign exchange revaluation and other	15	44
Balance at end of period	6,976	6,961
North American Personal P&C [Member]
Goodwill [Roll Forward]
Balance at beginning of period	2,235	196
Foreign exchange revaluation and other	5	14
Balance at end of period	2,240	2,235
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	4,817	2,078
Foreign exchange revaluation and other	187	(36)
Balance at end of period	5,004	4,817
Global Reinsurance
Goodwill [Roll Forward]
Balance at beginning of period	365	365
Foreign exchange revaluation and other	0	0
Balance at end of period	365	365
Life Insurance
Goodwill [Roll Forward]
Balance at beginning of period	820	820
Foreign exchange revaluation and other	2	0
Balance at end of period	$ 822	820
The Chubb Corporation [Member]
Goodwill [Roll Forward]
Acquisition		10,514
The Chubb Corporation [Member] | North America Commercial P&C Insurance
Goodwill [Roll Forward]
Acquisition		5,714
The Chubb Corporation [Member] | North American Personal P&C [Member]
Goodwill [Roll Forward]
Acquisition		2,025
The Chubb Corporation [Member] | North America Agricultural Insurance
Goodwill [Roll Forward]
Acquisition		0
The Chubb Corporation [Member] | Overseas General Insurance
Goodwill [Roll Forward]
Acquisition		2,775
The Chubb Corporation [Member] | Global Reinsurance
Goodwill [Roll Forward]
Acquisition		0
The Chubb Corporation [Member] | Life Insurance
Goodwill [Roll Forward]
Acquisition		$ 0