Postretirement benefits (Narrative) (Details) - USD ($) $ in Millions		3 Months Ended	12 Months Ended
	Oct. 31, 2016	Dec. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Defined Benefit Plan Disclosure [Line Items]
Expenses recognized during period under the defined contributions plans			$ 166	$ 150	$ 117
Available for sale, Fair Value		$ 80,115	78,939	80,115
Defined benefit plan, benefits paid inclusive of settlements			200	213
Defined Benefit Plan, Effect of One-Percentage Point Change in Assumed Health Care Cost Trend Rate [Abstract]
Effect of One Percentage Point Increase on Accumulated Postretirement Benefit Obligation			8
Effect of One Percentage Point Increase on Service and Interest Cost Components			1
Effect of One Percentage Point Decrease on Accumulated Postretirement Benefit Obligation			7
Effect of One Percentage Point Decrease on Service and Interest Cost Components			$ 1
Minimum
Defined Benefit Plan, Effect of One-Percentage Point Change in Assumed Health Care Cost Trend Rate [Abstract]
Defined Benefit Plan, Target Allocation Percentage			0.55
Maximum
Defined Benefit Plan, Effect of One-Percentage Point Change in Assumed Health Care Cost Trend Rate [Abstract]
Defined Benefit Plan, Target Allocation Percentage			0.65
Pension Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Accumulated Benefit Obligation		3,800	$ 4,300	3,800
Defined Benefit Plan, Expected Future Employer Contributions, Next Fiscal Year			28
Defined Benefit Plan, Plan Amendment [Abstract]
Defined Benefit Plan, Accumulated Benefit Obligation		3,800	4,300	3,800
Other Postretirement Benefits Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Fair Value of Plan Assets		159	157	159	0
Defined Benefit Plan, Expected Future Employer Contributions, Next Fiscal Year			2
Non-US [Member] | Pension Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Fair Value of Plan Assets		962	1,172	962	564
Future Amortization of (Gain) Loss			1
UNITED STATES
Defined Benefit Plan, Plan Amendment [Abstract]
Effect of Plan Amendment on Accumulated Benefit Obligation		496
UNITED STATES | Pension Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Fair Value of Plan Assets		2,765	3,109	2,765	$ 9
Defined Benefit Plan, Plan Amendment [Abstract]
Effect of Plan Amendment on Accumulated Benefit Obligation		113
UNITED STATES | Other Postretirement Benefits Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Future Amortization of Prior Service (Cost) Credit			80
UNITED STATES | Postretirement Health Coverage [Member]
Defined Benefit Plan, Plan Amendment [Abstract]
Effect of Plan Amendment on Accumulated Benefit Obligation	$ 383
Defined Benefit Plan, Effect of One-Percentage Point Change in Assumed Health Care Cost Trend Rate [Abstract]
Remaining Balance of Change in Benefit Obligation For Plan Amendment			267
UNITED STATES | Postretirement Health Coverage [Member] | Subject to Amortization [Member]
Defined Benefit Plan, Plan Amendment [Abstract]
Effect of Plan Amendment on Accumulated Benefit Obligation	$ 410	15	89
UNITED STATES | Postretirement Health Coverage [Member] | Subject to Amortization [Member] | The Chubb Corporation [Member]
Defined Benefit Plan, Plan Amendment [Abstract]
Effect of Plan Amendment on Accumulated Benefit Obligation			39
Foreign Government Debt Securities [Member] | Non-US [Member] | Pension Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Available for sale, Fair Value		435	456	435
Foreign Government Debt Securities [Member] | UNITED STATES | Pension Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Available for sale, Fair Value		487	692	487
Level 3 | Foreign Government Debt Securities [Member] | Non-US [Member] | Pension Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Available for sale, Fair Value		0	0	0
Level 3 | Foreign Government Debt Securities [Member] | UNITED STATES | Pension Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Available for sale, Fair Value		$ 5	$ 0	$ 5