Unpaid losses and loss expenses (Narrative) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2004
Liability for Claims and Claims Adjustment Expense [Line Items]
Prior Year Claims and Claims Adjustment Expense	$ (1,204)	$ (546)	$ (527)
Incurred activity	16,052	9,484	9,649
Aggregate reinsurance balances ceded by active ACE companies to Century	1,200
Liability for Claims and Claims Adjustment Expense	60,540	37,303	38,315	$ 37,443
Brandywine Run-off [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Incurred activity	512
Dividend retention fund established by INA Financial Corporation	50
Required minimum balance under the dividend retention fund	50
Contributions to the dividend retention fund					$ 35	$ 15
Minimum contribution from the dividend retention fund to Century not required for XOL agreement	200
Reinsurance coverage to Century provided by ACE INA under XOL	800
Statutory capital and surplus	25
Liability for Claims and Claims Adjustment Expense	2,000	1,900
Surplus note							$ 100
Westchester and Brandywine Run-off [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
NICO reinsurance protection on losses and loss expenses	438
NICO retention for losses and loss expenses incurred on or before 12/31/1996	$ 721
NICO pro-rata share of reinsurance protection (percent)	75.00%
NICO reinsurance protection on losses and loss expenses incurred on or before 12/31/1996, net of retenion	$ 1,000
North America Commercial P&C Insurance
Liability for Claims and Claims Adjustment Expense [Line Items]
Prior Year Claims and Claims Adjustment Expense	778	264	$ 378
Prior period net unpaid reserves represented by prior period development (percent)			1.40%
North America Commercial P&C Insurance | Long Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Prior Year Claims and Claims Adjustment Expense	650	155
North America Commercial P&C Insurance | Short Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Prior Year Claims and Claims Adjustment Expense	128	109
North America Commercial P&C Insurance | 2011 and prior [Member] | Directors and Officers Liability Insurance [Member] | Long Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Prior Year Claims and Claims Adjustment Expense	220
North America Commercial P&C Insurance | Accident Year 2010 and Prior [Member] | Commercial Excess and Umbrella [Member] | Long Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Prior Year Claims and Claims Adjustment Expense	264
North America Commercial P&C Insurance | Accident Year 2010 and Prior [Member] | Directors and Officers Liability Insurance [Member] | Long Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Prior Year Claims and Claims Adjustment Expense		108
North America Commercial P&C Insurance | Accident years 2012 and prior [Member] | Professional Errors and Omissions Insurance [Member] | Long Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Prior Year Claims and Claims Adjustment Expense	58
North America Commercial P&C Insurance | Short-duration Insurance Contracts, Accident Year 2010 [Member] | Auto Liability Excess Lines [Member] | Long Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Prior Year Claims and Claims Adjustment Expense		32
North America Commercial P&C Insurance | Short-duration Insurance Contracts, Accident Year 2010 [Member] | General Liability [Member] | Long Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Prior Year Claims and Claims Adjustment Expense		26
North America Commercial P&C Insurance | Accident year 2010 and 2011 [Member] | Commercial Excess and Umbrella [Member] | Long Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Prior Year Claims and Claims Adjustment Expense		(33)
North America Commercial P&C Insurance | Short-duration Insurance Contracts, Accident Year 2013 [Member] | Credit-related [Member] | Short Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Prior Year Claims and Claims Adjustment Expense		37
North America Commercial P&C Insurance | Short-duration Insurance Contracts, Accident Year 2013 [Member] | Excess Property Lines [Member] | Short Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Prior Year Claims and Claims Adjustment Expense		34
North America Commercial P&C Insurance | Accident year 2014 and 2015 [Member] | Property and Inland Marine [Member] | Short Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Prior Year Claims and Claims Adjustment Expense	79
North America Commercial P&C Insurance | Accident year 2012 and 2014 [Member] | Credit-related [Member] | Short Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Prior Year Claims and Claims Adjustment Expense	39
North America Commercial P&C Insurance - Workers' Compensation [Member] | Long Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Prior Year Claims and Claims Adjustment Expense	141	21
North America Commercial P&C Insurance - Workers' Compensation [Member] | Short-duration Insurance Contracts, Accident Year 2015 [Member] | Long Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Prior Year Claims and Claims Adjustment Expense	40
North America Commercial P&C Insurance - Workers' Compensation [Member] | Accident years 2012 and prior [Member] | Long Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Prior Year Claims and Claims Adjustment Expense	92
North America Commercial P&C Insurance - Workers' Compensation [Member] | Short-duration Insurance Contracts, Accident Year 2014 [Member] | Long Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Prior Year Claims and Claims Adjustment Expense		52
North America Commercial P&C Insurance - Workers' Compensation [Member] | Accident year 2009 and prior [Member] | Long Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Prior Year Claims and Claims Adjustment Expense		29
North America Personal P&C Insurance [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Prior Year Claims and Claims Adjustment Expense		(25)	$ (24)
Prior period net unpaid reserves represented by prior period development (percent)			0.10%
North America Personal P&C Insurance [Member] | Personal homeowner [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Prior Year Claims and Claims Adjustment Expense	(27)
North America Agricultural Insurance
Liability for Claims and Claims Adjustment Expense [Line Items]
Prior Year Claims and Claims Adjustment Expense	72	45	$ (34)
Global Reinsurance
Liability for Claims and Claims Adjustment Expense [Line Items]
Prior Year Claims and Claims Adjustment Expense	78	119	$ 63
Prior period net unpaid reserves represented by prior period development (percent)			0.20%
Global Reinsurance | Individual Legacy Liability [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Prior Year Claims and Claims Adjustment Expense		54
Global Reinsurance | Individual Legacy Liability [Member] | Long Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Prior Year Claims and Claims Adjustment Expense		42
Global Reinsurance | Individual Legacy Liability [Member] | Short Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Prior Year Claims and Claims Adjustment Expense		12
Global Reinsurance | 2011 and prior [Member] | Professional Errors and Omissions Insurance [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Prior Year Claims and Claims Adjustment Expense	30
Global Reinsurance | 2011 and prior [Member] | Casualty and Financial [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Prior Year Claims and Claims Adjustment Expense	$ 42
Global Reinsurance | Accident Year 2010 and Prior [Member] | Professional Errors and Omissions Insurance [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Prior Year Claims and Claims Adjustment Expense		33
Global Reinsurance | Accident year 2009 and prior [Member] | Casualty and Financial [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Prior Year Claims and Claims Adjustment Expense		23
Global Reinsurance | UNITED STATES
Liability for Claims and Claims Adjustment Expense [Line Items]
Loss by Geographic Percentage	70.00%
Global Reinsurance | UNITED STATES | Short-duration Insurance Contracts, Accident Year 2007 [Member] | Catastrophe [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Loss by Geographic Percentage	70.00%
Global Reinsurance | UNITED STATES | Short-duration Insurance Contracts, Accident Year 2007 [Member] | Non-catastrophe [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Loss by Geographic Percentage	50.00%
Global Reinsurance | UNITED STATES | Short-duration Insurance Contracts, Accident Year 2015 [Member] | Non-catastrophe [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Loss by Geographic Percentage	90.00%
Global Reinsurance | UNITED STATES | Accident year 2007 and after [Member] | Non-catastrophe [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Loss by Geographic Percentage	75.00%
Overseas General Insurance
Liability for Claims and Claims Adjustment Expense [Line Items]
Prior Year Claims and Claims Adjustment Expense	$ 423	343	$ 391
Prior period net unpaid reserves represented by prior period development (percent)			1.50%
Overseas General Insurance | Long Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Prior Year Claims and Claims Adjustment Expense	191	166
Overseas General Insurance | Short Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Prior Year Claims and Claims Adjustment Expense	232	177
Overseas General Insurance | Casualty and Financial [Member] | Long Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Prior Year Claims and Claims Adjustment Expense	165	140
Overseas General Insurance | Individual Legacy Liability [Member] | Long Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Prior Year Claims and Claims Adjustment Expense	25	26
Overseas General Insurance | 2011 and prior [Member] | Casualty and Financial [Member] | Long Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Prior Year Claims and Claims Adjustment Expense		175
Overseas General Insurance | Accident years 2012 and prior [Member] | Casualty and Financial [Member] | Long Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Prior Year Claims and Claims Adjustment Expense	257
Overseas General Insurance | Accident years 2012 and prior [Member] | Aviation [Member] | Short Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Prior Year Claims and Claims Adjustment Expense	28
Overseas General Insurance | Accident year 2013 to 2015 [Member] | Long Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Prior Year Claims and Claims Adjustment Expense	(92)
Overseas General Insurance | Accident year 2013 to 2015 [Member] | Accident and Health [Member] | Short Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Prior Year Claims and Claims Adjustment Expense	28
Overseas General Insurance | Accident year 2012 to 2014 [Member] | Property Insurance Product Line [Member] | Short Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Prior Year Claims and Claims Adjustment Expense	97
Overseas General Insurance | Accident year 2010 to 2014 [Member] | energy [Member] | Short Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Prior Year Claims and Claims Adjustment Expense	$ 43
Overseas General Insurance | Accident Year 2013 and prior [Member] | Property and Casualty [Member] | Short Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Prior Year Claims and Claims Adjustment Expense		90
Overseas General Insurance | Accident year 2013 and 2014 [Member] | Accident and Health [Member] | Short Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Prior Year Claims and Claims Adjustment Expense		34
Overseas General Insurance | Accident year 2012 and 2013 [Member] | Consumer [Member] | Short Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Prior Year Claims and Claims Adjustment Expense		26
Overseas General Insurance | Europe [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Loss by Geographic Percentage	40.00%
Overseas General Insurance | Europe [Member] | Accident year 2012 to 2014 [Member] | Casualty and Financial [Member] | Long Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Prior Year Claims and Claims Adjustment Expense		(35)
Overseas General Insurance - Non-Casualty [Member] | Latin America [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Loss by Geographic Percentage	35.00%
Corporate Segment [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Prior Year Claims and Claims Adjustment Expense			$ (247)
Prior period net unpaid reserves represented by prior period development (percent)			0.90%
Corporate Segment [Member] | Discontinued Operations [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Prior Year Claims and Claims Adjustment Expense	$ (48)	(30)
Corporate Segment [Member] | Westchester and Brandywine Run-off [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Prior Year Claims and Claims Adjustment Expense		(170)
Corporate Segment [Member] | Long Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Prior Year Claims and Claims Adjustment Expense	(189)	$ (200)
Corporate Segment [Member] | Asbestos & Environmental [Member] | Long Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Prior Year Claims and Claims Adjustment Expense	$ (141)