Unpaid losses and loss expenses (Narrative) (Details) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2004
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development		$ (395)	$ (792)	$ (896)
Prior Period Development, net Adjustments		$ 19	$ 53	$ 85
Prior Period Development Percentage Opening Net Unpaid Reserves	[1]	0.80%	1.60%	1.80%
Incurred activity		$ 21,710	$ 18,730	$ 18,067
Liability for Claims and Claims Adjustment Expense		67,811	62,690	62,960	$ 63,179
Brandywine Run-off [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Incurred activity		573
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		250	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		302	64
Aggregate reinsurance balances ceded by active ACE companies to Century		1,600	1,500
Liability for Claims and Claims Adjustment Expense		$ 2,100	1,800
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		372
Long Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development		(313)	(642)	(486)
Short Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development		(82)	(150)	(410)
North America Commercial P&C Insurance
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development		(702)	$ (649)	$ (610)
Prior Period Development, net Adjustments		$ 31
Prior Period Development Percentage Opening Net Unpaid Reserves	[1]	1.40%	1.30%	1.20%
North America Commercial P&C Insurance | Long Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development		$ (672)	$ (668)	$ (395)
Prior Period Development, net Adjustments		31
North America Commercial P&C Insurance | Short Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development		(30)	19	(215)
Prior Period Development, net Adjustments		0
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)
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		(26)	(34)
North America Commercial P&C Insurance | Other [Member] | Long Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development		77	26
North America Commercial P&C Insurance | Management Liability [Member] | Long Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development			(217)
North America Commercial P&C Insurance | Professional Liability [Member] | Long Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development			(60)
North America Commercial P&C Insurance | Workers' Compensation Insurance [Member] | Long Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development		(363)
North America Commercial P&C Insurance | Property and Inland Marine | 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 | 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 2014 and prior [Member] | Commercial Excess and Umbrella [Member] | Long Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development		(83)
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 Year 2013 and prior [Member] | 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 | Short-duration Insurance Contracts, Accident Year 2016 [Member] | Foreign Casualty Line [Member] | Long Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development		(40)
North America Commercial P&C Insurance | Short-duration Insurance Contracts, Accident Year 2017 [Member] | Surety Product Line [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 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] | Surety Product Line [Member] | Short Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development		(31)
North America Commercial P&C Insurance | Short-duration Insurance Contracts, Accident Year 2018 [Member] | Property and Inland Marine | 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 2016 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		(67)
North America Commercial P&C Insurance | Accident years 2016 and prior [Member] | Multi-Line [Member] | Long Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development		(41)
North America Commercial P&C Insurance | Accident years 2016 and prior [Member] | Environmental lines [Member] | Long Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development		(43)
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] | Medical Portfolios [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] | 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] | Management Liability [Member] | Long Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development		(231)
North America Commercial P&C Insurance | Accident years 2016 - 2019 [Member] | Medical Portfolios [Member] | Long Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development		64
North America Commercial P&C Insurance | Accident years 2018 - 2019 [Member] | Accident and Health [Member] | Short Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development		(37)
North America Commercial P&C Insurance | Short-Duration Insurance Contract, Accident Year 2019 [Member] | Workers' Compensation Insurance [Member] | Long Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development		(62)
North America Commercial P&C Insurance | Short-Duration Insurance Contract, Accident Year 2019 [Member] | Property and Inland Marine | Short Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development		21
North America Commercial P&C Insurance | Accident years 2014 - 2018 [Member] | 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 | Accident years 2017 - 2019 | General Liability Coverages | Long Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development		48
North America Commercial P&C Insurance | Accident year 2017 and prior | Non-catastrophe [Member] | Property and Inland Marine | 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 2017 - 2018 | Catastrophe [Member] | Property and Inland Marine | Short Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development			(41)
North America Personal P&C Insurance [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development		$ 63	$ (95)	$ 41
Prior Period Development Percentage Opening Net Unpaid Reserves	[1]	0.10%	0.20%	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		63	(95)	41
Prior Period Development, net Adjustments		(18)
North America Personal P&C Insurance [Member] | Personal Excess [Member] | Short Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development		(22)
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 Excess [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 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 Personal P&C Insurance [Member] | Accident Year 2017 to 2019 [Member] | Homeowners and Valuables [Member] | Short Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development		84
North America Agricultural Insurance
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development		$ (10)	$ (80)	$ (110)
Prior Period Development Percentage Opening Net Unpaid Reserves	[1]	0.00%	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		(10)	(80)	(110)
Prior Period Development, net Adjustments		9
Overseas General Insurance
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development		(150)	$ (92)	$ (212)
Prior Period Development, net Adjustments		$ 0
Prior Period Development Percentage Opening Net Unpaid Reserves	[1]	0.30%	0.20%	0.40%
Overseas General Insurance | Long Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development		$ (49)	$ (68)	$ (67)
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		(101)	(24)	(145)
Prior Period Development, net Adjustments		0
Overseas General Insurance | Casualty Lines [Member] | Long Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development		(94)	(101)
Overseas General Insurance | Financial [Member] | Long Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development		80	52
Overseas General Insurance | Accident years 2017 - 2018 [Member] | Surety Product Line [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] | Casualty Lines [Member] | Long Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development		(143)
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		(61)	(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 | Accident Year 2017 to 2019 [Member] | Casualty Lines [Member] | Long Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development		49
Overseas General Insurance | Accident years 2016 - 2019 [Member] | Financial [Member] | Long Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development		141
Overseas General Insurance | Accident years 2018 - 2019 [Member] | Accident and Health [Member] | Short Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development		(21)
Overseas General Insurance | Accident years 2018 - 2019 [Member] | Political Risk | Long Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development		(22)
Overseas General Insurance | Accident years 2012 through 2019 | Marine [Member] | Short Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development		$ (69)
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%
Global Reinsurance Non-Casualty [Member] | Accident year 2013 to 2020
Liability for Claims and Claims Adjustment Expense [Line Items]
Loss by Geographic Percentage		82.00%
Global Reinsurance Non-Casualty [Member] | Accident years 2011 to 2012
Liability for Claims and Claims Adjustment Expense [Line Items]
Loss by Geographic Percentage		59.00%
Global Reinsurance Non-Casualty [Member] | accident year 2011 and after
Liability for Claims and Claims Adjustment Expense [Line Items]
Loss by Geographic Percentage		77.00%
Segment Global Reinsurance [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development		$ (29)	$ (29)	$ (50)
Prior Period Development, net Adjustments		$ (3)
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		$ (25)	$ (59)	$ (69)
Prior Period Development, net Adjustments		(2)
Segment Global Reinsurance [Member] | Short Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development		(4)	30	19
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)
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
Corporate Segment [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development		$ 433	$ 153	$ 45
Prior Period Development Percentage Opening Net Unpaid Reserves	[1]	0.90%	0.30%	0.10%
Corporate Segment [Member] | Discontinued Operations [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development		$ 38	$ 37
Corporate Segment [Member] | Long Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development		433	153	$ 45
Prior Period Development, net Adjustments		0
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] | Other [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development		254
Corporate Segment [Member] | Other [Member] | Other [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development		27
Corporate Segment [Member] | Asbestos & Environmental [Member] | Long Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development		$ 106	$ 116

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