Goodwill and other intangible assets Intangible Assets (Estimated Amortization Expense Over the Next Five Years (Details) $ in Millions	Mar. 31, 2016 USD ($)
Finite-Lived Intangible Assets [Line Items]
Finite-Lived Intangible Assets, Amortization Expense Next Three Months	$ 1
Finite-Lived Intangible Assets Amortization Expense Next Three to Six Months	1
Finite-Lived Intangible Assets, Amortization Expense Next Six to Nine Months	1
Finite-Lived Intangible Assets, Amortization Expense, Next Twelve Months	244
Finite-Lived Intangible Assets, Amortization Expense, Year Two	321
Finite-Lived Intangible Assets, Amortization Expense, Year Three	281
Finite-Lived Intangible Assets, Amortization Expense, Year Four	262
Finite-Lived Intangible Assets, Amortization Expense, Year Five	248
FiniteLivedIntangibleAssetsAmortizationExpenseTotal	1,359
The Chubb Corporation [Member]
Finite-Lived Intangible Assets [Line Items]
Finite-Lived Intangible Assets, Amortization Expense Next Three Months	(20)
Finite-Lived Intangible Assets Amortization Expense Next Three to Six Months	(20)
Finite-Lived Intangible Assets, Amortization Expense Next Six to Nine Months	(20)
Finite-Lived Intangible Assets, Amortization Expense, Next Twelve Months	165
Finite-Lived Intangible Assets, Amortization Expense, Year Two	251
Finite-Lived Intangible Assets, Amortization Expense, Year Three	217
Finite-Lived Intangible Assets, Amortization Expense, Year Four	204
Finite-Lived Intangible Assets, Amortization Expense, Year Five	196
FiniteLivedIntangibleAssetsAmortizationExpenseTotal	973
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 Months	33
Finite-Lived Intangible Assets Amortization Expense Next Three to Six Months	33
Finite-Lived Intangible Assets, Amortization Expense Next Six to Nine Months	33
Finite-Lived Intangible Assets, Amortization Expense, Next Twelve Months	293
Finite-Lived Intangible Assets, Amortization Expense, Year Two	320
Finite-Lived Intangible Assets, Amortization Expense, Year Three	279
Finite-Lived Intangible Assets, Amortization Expense, Year Four	239
Finite-Lived Intangible Assets, Amortization Expense, Year Five	216
FiniteLivedIntangibleAssetsAmortizationExpenseTotal	1,446
Internally Developed Technology [Member] | The Chubb Corporation [Member]
Finite-Lived Intangible Assets [Line Items]
Finite-Lived Intangible Assets, Amortization Expense Next Three Months	8
Finite-Lived Intangible Assets Amortization Expense Next Three to Six Months	8
Finite-Lived Intangible Assets, Amortization Expense Next Six to Nine 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	88
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 Months	(61)
Finite-Lived Intangible Assets Amortization Expense Next Three to Six Months	(61)
Finite-Lived Intangible Assets, Amortization Expense Next Six to Nine Months	(61)
Finite-Lived Intangible Assets, Amortization Expense, Next Twelve Months	(160)
Finite-Lived Intangible Assets, Amortization Expense, Year Two	(101)
Finite-Lived Intangible Assets, Amortization Expense, Year Three	(62)
Finite-Lived Intangible Assets, Amortization Expense, Year Four	(35)
Finite-Lived Intangible Assets, Amortization Expense, Year Five	(20)
FiniteLivedIntangibleAssetsAmortizationExpenseTotal	(561)
Other Intangible Assets [Member]
Finite-Lived Intangible Assets [Line Items]
Finite-Lived Intangible Assets, Amortization Expense Next Three Months	21
Finite-Lived Intangible Assets Amortization Expense Next Three to Six Months	21
Finite-Lived Intangible Assets, Amortization Expense Next Six to Nine Months	21
Finite-Lived Intangible Assets, Amortization Expense, Next Twelve Months	79
Finite-Lived Intangible Assets, Amortization Expense, Year Two	70
Finite-Lived Intangible Assets, Amortization Expense, Year Three	64
Finite-Lived Intangible Assets, Amortization Expense, Year Four	58
Finite-Lived Intangible Assets, Amortization Expense, Year Five	52
FiniteLivedIntangibleAssetsAmortizationExpenseTotal	$ 386