Postretirement benefits Schedule of Net Funded Status (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Pension Plan [Member] | UNITED STATES
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Benefit Obligation, beginning of year	$ 3,035	$ 10
Acquisition of Chubb Corp, Benefit obligation	0	3,153
Service Cost	63	75	$ 0
Interest Cost	105	103	0
Actuarial Loss	232	131
Defined Benefit Plan, Benefit Obligation, Benefits Paid	(132)	(79)
Amendments	0	0
Curtailments	0	(259)
Settlements, Benefit obligations	(18)	(99)
Foreign currency revaluation, benefit obligations	0	0
Benefit Obligation, end of year	3,285	3,035	10
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Plan assets at fair value, beginning of year	2,765	9
Acquisition of Chubb Corp, Plan Assets	0	2,473
Actual Return on Plan Assets	441	359
Employer Contributions	53	98
Benefits Paid	(132)	(79)
Settlements, Plan Assets	(18)	(95)
Foreign currency revaluation, Plan Assets	0	0
Plan assets at fair value, end of year	3,109	2,765	9
Defined Benefit Plan, Funded (Unfunded) Status of Plan	(176)	(270)
Pension Plan [Member] | Non-US [Member]
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Benefit Obligation, beginning of year	1,025	559
Acquisition of Chubb Corp, Benefit obligation	0	372
Service Cost	17	18	6
Interest Cost	27	30	21
Actuarial Loss	(4)	204
Defined Benefit Plan, Benefit Obligation, Benefits Paid	(28)	(22)
Amendments	0	(9)
Curtailments	(32)	(7)
Settlements, Benefit obligations	(8)	(7)
Foreign currency revaluation, benefit obligations	80	(113)
Benefit Obligation, end of year	1,077	1,025	559
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Plan assets at fair value, beginning of year	962	564
Acquisition of Chubb Corp, Plan Assets	0	315
Actual Return on Plan Assets	100	168
Employer Contributions	63	67
Benefits Paid	(28)	(22)
Settlements, Plan Assets	(8)	(7)
Foreign currency revaluation, Plan Assets	83	(123)
Plan assets at fair value, end of year	1,172	962	564
Defined Benefit Plan, Funded (Unfunded) Status of Plan	95	(63)
Other Postretirement Benefits Plan [Member]
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Benefit Obligation, beginning of year	165	16
Acquisition of Chubb Corp, Benefit obligation	0	506
Service Cost	2	10	1
Interest Cost	4	17	0
Actuarial Loss	(2)	36
Defined Benefit Plan, Benefit Obligation, Benefits Paid	(14)	(11)
Amendments	(23)	(410)
Curtailments	2	0
Settlements, Benefit obligations	0	0
Foreign currency revaluation, benefit obligations	3	1
Benefit Obligation, end of year	137	165	16
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Plan assets at fair value, beginning of year	159	0
Acquisition of Chubb Corp, Plan Assets	0	138
Actual Return on Plan Assets	6	29
Employer Contributions	6	3
Benefits Paid	(14)	(11)
Settlements, Plan Assets	0	0
Foreign currency revaluation, Plan Assets	0	0
Plan assets at fair value, end of year	157	159	$ 0
Defined Benefit Plan, Funded (Unfunded) Status of Plan	$ 20	$ (6)