Postretirement benefits Schedule of Net Funded Status (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Pension Plan [Member]
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Service Cost	$ 93	$ 6	$ 6
Interest Cost	133	21	25
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Plan assets at fair value, beginning of year	573
Plan assets at fair value, end of year		573
Pension Plan [Member] | UNITED STATES
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Benefit Obligation, beginning of year	10	10
Acquisition of Chubb Corp, Benefit obligation	3,153	0
Service Cost	75	0	0
Interest Cost	103	0	0
Actuarial Loss	131	0
Benefits Paid	(79)	0
Amendments	0	0
Curtailments	(259)	0
Settlements, Benefit obligations	(99)	0
Foreign currency revaluation, benefit obligations	0	0
Benefit Obligation, end of year	3,035	10	10
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Plan assets at fair value, beginning of year	9	9
Acquisition of Chubb Corp, Plan Assets	2,473	0
Actual Return on Plan Assets	359	0
Employer Contributions	98	0
Benefits Paid	(79)	0
Settlements, Plan Assets	(95)	0
Foreign currency revaluation, Plan Assets	0	0
Plan assets at fair value, end of year	2,765	9	9
Defined Benefit Plan, Funded Status of Plan	(270)	(1)
Pension Plan [Member] | Non-US [Member]
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Benefit Obligation, beginning of year	559	584
Acquisition of Chubb Corp, Benefit obligation	372	0
Service Cost	18	6	6
Interest Cost	30	21	25
Actuarial Loss	204	13
Benefits Paid	(22)	(22)
Amendments	(9)	1
Curtailments	(7)	0
Settlements, Benefit obligations	(7)	(7)
Foreign currency revaluation, benefit obligations	(113)	(37)
Benefit Obligation, end of year	1,025	559	584
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Plan assets at fair value, beginning of year	564	579
Acquisition of Chubb Corp, Plan Assets	315	0
Actual Return on Plan Assets	168	40
Employer Contributions	67	10
Benefits Paid	(22)	(22)
Settlements, Plan Assets	(7)	(7)
Foreign currency revaluation, Plan Assets	(123)	(36)
Plan assets at fair value, end of year	962	564	579
Defined Benefit Plan, Funded Status of Plan	(63)	5
Other Postretirement Benefit Plan [Member]
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Service Cost	10	1	1
Interest Cost	17	0	0
Other Postretirement Benefit Plan [Member] | UNITED STATES
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Benefit Obligation, beginning of year	16	16
Acquisition of Chubb Corp, Benefit obligation	491	0
Service Cost	9	1	1
Interest Cost	16	0	0
Actuarial Loss	33	(1)
Benefits Paid	(11)	0
Amendments	(410)	0
Curtailments	0	0
Settlements, Benefit obligations	0	0
Foreign currency revaluation, benefit obligations	0	0
Benefit Obligation, end of year	144	16	16
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Plan assets at fair value, beginning of year	0	0
Acquisition of Chubb Corp, Plan Assets	138	0
Actual Return on Plan Assets	29	0
Employer Contributions	3	0
Benefits Paid	(11)	0
Settlements, Plan Assets	0	0
Foreign currency revaluation, Plan Assets	0	0
Plan assets at fair value, end of year	159	0	0
Defined Benefit Plan, Funded Status of Plan	15	(16)
Other Postretirement Benefit Plan [Member] | Non-US [Member]
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Benefit Obligation, beginning of year	0	0
Acquisition of Chubb Corp, Benefit obligation	15	0
Service Cost	1	0	0
Interest Cost	1	0	0
Actuarial Loss	3	0
Benefits Paid	0	0
Amendments	0	0
Curtailments	0	0
Settlements, Benefit obligations	0	0
Foreign currency revaluation, benefit obligations	1	0
Benefit Obligation, end of year	21	0	0
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Plan assets at fair value, beginning of year	0	0
Acquisition of Chubb Corp, Plan Assets	0	0
Actual Return on Plan Assets	0	0
Employer Contributions	0	0
Benefits Paid	0	0
Settlements, Plan Assets	0	0
Foreign currency revaluation, Plan Assets	0	0
Plan assets at fair value, end of year	0	0	$ 0
Defined Benefit Plan, Funded Status of Plan	$ (21)	$ 0