Goodwill and other intangible assets Intangible Assets (Estimated Amortization Expense Over the Next Five Years (Details) $ in Millions	Sep. 30, 2016 USD ($)
Finite-Lived Intangible Assets [Line Items]
Finite-Lived Intangible Assets Amortization Expense Next Three to Six Months	$ 3
Finite-Lived Intangible Assets, Amortization Expense, Next Twelve Months	252
Finite-Lived Intangible Assets, Amortization Expense, Year Two	327
Finite-Lived Intangible Assets, Amortization Expense, Year Three	285
Finite-Lived Intangible Assets, Amortization Expense, Year Four	263
Finite-Lived Intangible Assets, Amortization Expense, Year Five	250
FiniteLivedIntangibleAssetsAmortizationExpenseTotal	1,380
The Chubb Corporation [Member]
Finite-Lived Intangible Assets [Line Items]
Unearned Premium Reserves Amortization, Next Three to Six Months	144
Finite-Lived Intangible Assets Amortization Expense Next Three to Six Months	(20)
Finite-Lived Intangible Assets, Amortization Expense, Next Twelve Months	167
Finite-Lived Intangible Assets, Amortization Expense, Year Two	254
Finite-Lived Intangible Assets, Amortization Expense, Year Three	219
Finite-Lived Intangible Assets, Amortization Expense, Year Four	204
Finite-Lived Intangible Assets, Amortization Expense, Year Five	197
FiniteLivedIntangibleAssetsAmortizationExpenseTotal	1,021
Agency distribution relationships and renewal rights [Member] | The Chubb Corporation [Member]
Finite-Lived Intangible Assets [Line Items]
Finite-Lived Intangible Assets Amortization Expense Next Three to Six Months	33
Finite-Lived Intangible Assets, Amortization Expense, Next Twelve Months	296
Finite-Lived Intangible Assets, Amortization Expense, Year Two	324
Finite-Lived Intangible Assets, Amortization Expense, Year Three	281
Finite-Lived Intangible Assets, Amortization Expense, Year Four	240
Finite-Lived Intangible Assets, Amortization Expense, Year Five	217
FiniteLivedIntangibleAssetsAmortizationExpenseTotal	1,391
Internally Developed Technology [Member] | The Chubb Corporation [Member]
Finite-Lived Intangible Assets [Line Items]
Finite-Lived Intangible Assets Amortization Expense Next Three to Six Months	8
Finite-Lived Intangible Assets, Amortization Expense, Next Twelve Months	32
Finite-Lived Intangible Assets, Amortization Expense, Year Two	32
Finite-Lived Intangible Assets, Amortization Expense, Year Three	0
Finite-Lived Intangible Assets, Amortization Expense, Year Four	0
Finite-Lived Intangible Assets, Amortization Expense, Year Five	0
FiniteLivedIntangibleAssetsAmortizationExpenseTotal	72
Fair Value Adjustment to Acquired Loss Reserves [Member] | The Chubb Corporation [Member]
Finite-Lived Intangible Assets [Line Items]
Finite-Lived Intangible Assets Amortization Expense Next Three to Six Months	(61)
Finite-Lived Intangible Assets, Amortization Expense, Next Twelve Months	(161)
Finite-Lived Intangible Assets, Amortization Expense, Year Two	(102)
Finite-Lived Intangible Assets, Amortization Expense, Year Three	(62)
Finite-Lived Intangible Assets, Amortization Expense, Year Four	(36)
Finite-Lived Intangible Assets, Amortization Expense, Year Five	(20)
FiniteLivedIntangibleAssetsAmortizationExpenseTotal	(442)
Other Intangible Assets [Member]
Finite-Lived Intangible Assets [Line Items]
Finite-Lived Intangible Assets Amortization Expense Next Three to Six Months	23
Finite-Lived Intangible Assets, Amortization Expense, Next Twelve Months	85
Finite-Lived Intangible Assets, Amortization Expense, Year Two	73
Finite-Lived Intangible Assets, Amortization Expense, Year Three	66
Finite-Lived Intangible Assets, Amortization Expense, Year Four	59
Finite-Lived Intangible Assets, Amortization Expense, Year Five	53
FiniteLivedIntangibleAssetsAmortizationExpenseTotal	$ 359