Unpaid losses and loss expenses (Narrative) (Details) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2004
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development		$ (792)	$ (896)	$ (829)
Prior Period Development, net Adjustments		$ 53	$ 85	$ 108
Prior Period Development Percentage Opening Net Unpaid Reserves	[1]	1.60%	1.80%	1.70%
Incurred activity		$ 18,730	$ 18,067	$ 18,454
Liability for Claims and Claims Adjustment Expense		62,690	62,960	63,179	$ 60,540
Brandywine Run-off [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Incurred activity		622
Reinsurance coverage to Century provided by ACE INA under XOL		800
Statutory capital and surplus		25
Dividend retention fund established by INA Financial Corporation		50
Required minimum balance under the dividend retention fund		50
Contributions to the dividend retention fund		90	50			$ 35	$ 15
Minimum contribution from the dividend retention fund to Century not required for XOL agreement		200
Dividend Retention Fund Contribution to XOL		64	39
Aggregate reinsurance balances ceded by active ACE companies to Century		1,500	1,500
Liability for Claims and Claims Adjustment Expense		$ 1,800	2,000
Surplus note								$ 100
Westchester and Brandywine Run-off [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
NICO pro-rata share of reinsurance protection (percent)		75.00%
NICO retention for losses and loss expenses incurred on or before 12/31/1996		$ 721
NICO reinsurance protection on losses and loss expenses incurred on or before 12/31/1996, net of retenion		1,000
NICO reinsurance protection on losses and loss expenses		384
Long Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development		(642)	(486)	(423)
Short Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development		(150)	(410)	(406)
North America Commercial P&C Insurance
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development		(649)	$ (610)	$ (746)
Prior Period Development, net Adjustments		$ 34
Prior Period Development Percentage Opening Net Unpaid Reserves	[1]	1.30%	1.20%	1.60%
North America Commercial P&C Insurance | Long Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development		$ (668)	$ (395)	$ (562)
Prior Period Development, net Adjustments		39
North America Commercial P&C Insurance | Short Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development		19	(215)	(184)
Prior Period Development, net Adjustments		(5)
North America Commercial P&C Insurance | North America Commercial P&C Insurance - Workers' Compensation [Member] | Long Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development		(303)	(194)
North America Commercial P&C Insurance | Commercial Excess and Umbrella [Member] | Long Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development		(41)
North America Commercial P&C Insurance | Multi-Line [Member] | Long Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development		(34)
North America Commercial P&C Insurance | Credit-related [Member] | Short Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development			(19)
North America Commercial P&C Insurance | Other [Member] | Long Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development		26
North America Commercial P&C Insurance | Other [Member] | Short Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development			(60)
North America Commercial P&C Insurance | Commercial Property and Marine [Member] | Short Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development			(155)
North America Commercial P&C Insurance | Auto Liability Excess Lines [Member] | Long Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development		38
North America Commercial P&C Insurance | Non-catastrophe [Member] | Property and Inland Marine [Member] | Short Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development		108
North America Commercial P&C Insurance | Catastrophe [Member] | Property and Inland Marine [Member] | Short Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development		(36)
North America Commercial P&C Insurance | Accident years 2015 through 2017 [Member] | Medical Portfolios [Member] | Long Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development			91
North America Commercial P&C Insurance | Accident years 2015 through 2017 [Member] | Other [Member] | Long Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development			109
North America Commercial P&C Insurance | Accident years 2014 and prior [Member] | North America Commercial P&C Insurance - Workers' Compensation [Member] | Long Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development			(138)
North America Commercial P&C Insurance | Accident years 2014 and prior [Member] | Foreign Casualty Line [Member] | Long Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development			(28)
North America Commercial P&C Insurance | Short-duration Insurance Contracts, Accident Year 2015 [Member] | Political [Member] | Long Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development		(23)
North America Commercial P&C Insurance | Accident years 2012 and prior [Member] | Commercial Excess and Umbrella [Member] | Long Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development			(100)
North America Commercial P&C Insurance | Accident years 2002 and prior [Member] | Professional Liability [Member] | Long Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development			(33)
North America Commercial P&C Insurance | Short-duration Insurance Contracts, Accident Year 2014 [Member] | Political [Member] | Long Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development			(23)
North America Commercial P&C Insurance | Accident Year 2013 and prior [Member] | Management Liability [Member] | Long Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development			(199)
North America Commercial P&C Insurance | Accident years 2017 - 2018 [Member] | Catastrophe [Member] | Property and Inland Marine [Member] | Short Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development		(41)
North America Commercial P&C Insurance | Short-duration Insurance Contracts, Accident Year 2017 [Member] | North America Commercial P&C Insurance - Workers' Compensation [Member] | Long Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development			(56)
North America Commercial P&C Insurance | Short-duration Insurance Contracts, Accident Year 2017 [Member] | Credit-related [Member] | Short Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development		(49)
North America Commercial P&C Insurance | Short-duration Insurance Contracts, Accident Year 2017 [Member] | Catastrophe [Member] | Commercial Property and Marine [Member] | Short Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development			(129)
North America Commercial P&C Insurance | Short-duration Insurance Contracts, Accident Year 2018 [Member] | North America Commercial P&C Insurance - Workers' Compensation [Member] | Long Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development		(61)
North America Commercial P&C Insurance | Short-duration Insurance Contracts, Accident Year 2018 [Member] | Property and Inland Marine [Member] | Short Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development		152
North America Commercial P&C Insurance | Accident years 2017 and prior [Member] | Non-catastrophe [Member] | Property and Inland Marine [Member] | Short Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development		(44)
North America Commercial P&C Insurance | Accident Year 2015 and prior [Member] | Professional Errors and Omissions Insurance [Member] | Long Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development		(60)
North America Commercial P&C Insurance | Accident Year 2015 and prior [Member] | Foreign Casualty Line [Member] | Long Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development		(39)
North America Commercial P&C Insurance | Accident Year 2015 and prior [Member] | Multi-Line [Member] | Long Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development		(36)
North America Commercial P&C Insurance | Accident Year 2015 and prior [Member] | Medical and Life Sciences [Member] | Long Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development		(24)
North America Commercial P&C Insurance | Accident Year 2015 and prior [Member] | Management Liability [Member] | Long Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development		(217)
North America Personal P&C Insurance [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development		$ (95)	$ 41	$ 69
Prior Period Development Percentage Opening Net Unpaid Reserves	[1]	0.20%	0.10%	0.10%
North America Personal P&C Insurance [Member] | Long Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development		$ 0	$ 0	$ 0
North America Personal P&C Insurance [Member] | Short Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development		(95)	41	69
Prior Period Development, net Adjustments		(4)
North America Personal P&C Insurance [Member] | Auto Liability and Marine [Member] | Short Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development		(16)
North America Personal P&C Insurance [Member] | Short-duration Insurance Contracts, Accident Year 2015 [Member] | Personal Excess [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development			(24)
North America Personal P&C Insurance [Member] | Accident years 2017 - 2018 [Member] | Catastrophe [Member] | Short Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development		(132)
North America Personal P&C Insurance [Member] | Short-duration Insurance Contracts, Accident Year 2016 [Member] | Personal lines [Member] | Short Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development		(26)
North America Personal P&C Insurance [Member] | Short-duration Insurance Contracts, Accident Year 2017 [Member] | Homeowners and Valuables Lines [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development			63
North America Personal P&C Insurance [Member] | Short-duration Insurance Contracts, Accident Year 2017 [Member] | Catastrophe [Member] | Homeowners and Valuables Lines [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development			(73)
North America Personal P&C Insurance [Member] | Short-duration Insurance Contracts, Accident Year 2017 [Member] | Excluding Catastrophic Events [Member] | Homeowners and Valuables Lines [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development			136
North America Personal P&C Insurance [Member] | Short-duration Insurance Contracts, Accident Year 2018 [Member] | Homeowners and Valuables [Member] | Short Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development		82
North America Agricultural Insurance
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development		$ (80)	$ (110)	$ (119)
Prior Period Development Percentage Opening Net Unpaid Reserves	[1]	0.20%	0.20%	0.20%
North America Agricultural Insurance | Long Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development		$ 0	$ 0	$ 0
North America Agricultural Insurance | Short Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development		(80)	(110)	(119)
Prior Period Development, net Adjustments		23
Overseas General Insurance
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development		(92)	$ (212)	$ (252)
Prior Period Development, net Adjustments		$ 1
Prior Period Development Percentage Opening Net Unpaid Reserves	[1]	0.20%	0.40%	0.50%
Overseas General Insurance | Long Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development		$ (68)	$ (67)	$ (71)
Prior Period Development, net Adjustments		0
Overseas General Insurance | Short Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development		(24)	(145)	(181)
Prior Period Development, net Adjustments		1
Overseas General Insurance | Other [Member] | Long Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development			(32)
Overseas General Insurance | Casualty Lines [Member] | Long Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development		(101)
Overseas General Insurance | Financial [Member] | Long Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development		52	38
Overseas General Insurance | Casualty [Member] | Long Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development			(70)
Overseas General Insurance | Aviation [Member] | Long Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development			(10)
Overseas General Insurance | Political [Member] | Long Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development			(12)
Overseas General Insurance | Accident years 2015 through 2017 [Member] | Financial [Member] | Long Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development			131
Overseas General Insurance | Accident years 2015 through 2017 [Member] | Casualty [Member] | Long Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development			38
Overseas General Insurance | Accident years 2015 through 2017 [Member] | Accident and Health [Member] | Short Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development			(33)
Overseas General Insurance | Accident years 2013 - 2016 [Member] | Property and Inland Marine [Member] | Short Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development			(99)
Overseas General Insurance | Accident years 2014 and prior [Member] | Financial [Member] | Long Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development			(93)
Overseas General Insurance | Accident years 2014 and prior [Member] | Casualty [Member] | Long Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development			(107)
Overseas General Insurance | Accident years 2017 - 2018 [Member] | Credit-related [Member] | Short Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development		27
Overseas General Insurance | Accident years 2017 - 2018 [Member] | Accident and Health [Member] | Short Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development		(45)
Overseas General Insurance | Short-duration Insurance Contracts, Accident Year 2018 [Member] | Construction [Member] | Short Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development		23
Overseas General Insurance | Accident years 2016 and prior [Member] | Marine [Member] | Short Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development		(36)
Overseas General Insurance | Accident Year 2015 and prior [Member] | Casualty Lines [Member] | Long Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development		(123)
Overseas General Insurance | Accident Year 2015 and prior [Member] | Financial [Member] | Long Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development		(75)
Overseas General Insurance | Accident years 2016 - 2018 [Member] | Casualty Lines [Member] | Long Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development		22
Overseas General Insurance | Accident years 2016 - 2018 [Member] | Financial [Member] | Long Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development		$ 127
Overseas General Insurance - Casualty [Member] | Europe [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Loss by Geographic Percentage		45.00%
Overseas General Insurance - Non-Casualty [Member] | Latin America [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Loss by Geographic Percentage		30.00%
Overseas General Insurance - Non-Casualty [Member] | Europe [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Loss by Geographic Percentage		30.00%
Global Reinsurance Non-Casualty [Member] | Accident year 2010 and after [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Loss by Geographic Percentage		73.00%
Global Reinsurance Non-Casualty [Member] | Accident years 2010 to 2012 [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Loss by Geographic Percentage		58.00%
Global Reinsurance Non-Casualty [Member] | Accident years 2013 to 2019 [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Loss by Geographic Percentage		80.00%
Segment Global Reinsurance [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development		$ (29)	$ (50)	$ (59)
Prior Period Development, net Adjustments		$ (1)
Prior Period Development Percentage Opening Net Unpaid Reserves	[1]	0.10%	0.10%	0.10%
Segment Global Reinsurance [Member] | Long Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development		$ (59)	$ (69)	$ (68)
Prior Period Development, net Adjustments		0
Segment Global Reinsurance [Member] | Short Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development		30	19	9
Prior Period Development, net Adjustments		(1)
Segment Global Reinsurance [Member] | Accident Year 2013 and prior [Member] | Other [Member] | Long Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development		(59)	(69)
Segment Global Reinsurance [Member] | Accident years 2017 - 2018 [Member] | Catastrophe [Member] | Short Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development		44
Segment Global Reinsurance [Member] | Short-duration Insurance Contracts, Accident Year 2017 [Member] | Catastrophe [Member] | Short Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development			18
Corporate Segment [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development		$ 153	$ 45	$ 278
Prior Period Development Percentage Opening Net Unpaid Reserves	[1]	0.30%	0.10%	0.60%
Corporate Segment [Member] | Discontinued Operations [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development		$ 37
Corporate Segment [Member] | Long Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development		153	$ 45	$ 278
Prior Period Development, net Adjustments		0
Corporate Segment [Member] | Long Tail [Member] | Discontinued Operations [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development			35
Corporate Segment [Member] | Long Tail [Member] | Reinsurance, Structured Settlements [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development			(205)
Corporate Segment [Member] | Short Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development		0	0	$ 0
Corporate Segment [Member] | Asbestos & Environmental [Member] | Long Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development		$ 116	216
Environmental Issue [Member] | Overseas General Insurance | Long Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development			$ (10)

[1] Calculated based on the beginning of period consolidated net unpaid losses and loss expenses.