Unpaid losses and loss expenses Unpaid losses and loss expenses (Narrative) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development	$ (192)	$ (118)
Long Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development	(37)	(32)
Short Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development	(155)	(86)
North America Commercial P&C Insurance [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development	(127)	(105)
North America Commercial P&C Insurance [Member] | Long Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development	(46)	(43)
North America Commercial P&C Insurance [Member] | Short Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development	(81)	(62)
North America Commercial P&C Insurance [Member] | Professional Malpractice Liability Insurance [Member] | Long Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development	(51)
North America Commercial P&C Insurance [Member] | Environmental lines [Member] | Long Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development	(35)
North America Commercial P&C Insurance [Member] | Commercial Excess and Umbrella [Member] | Long Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development	57	49
North America Commercial P&C Insurance [Member] | General Liability [Member] | Long Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development		23
North America Commercial P&C Insurance [Member] | Accident years 2016 and prior [Member] | Professional Malpractice Liability Insurance [Member] | Long Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development		(66)
North America Commercial P&C Insurance [Member] | 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 [Member] | Accident years 2016 and prior [Member] | Workers' Compensation Insurance [Member] | Long Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development		(28)
North America Commercial P&C Insurance [Member] | Short-duration Insurance Contracts, Accident Year 2018 [Member] | Surety Product Line [Member] | Short Tail [Member] | Catastrophe [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development		(31)
North America Commercial P&C Insurance [Member] | Accident years 2018 - 2019 [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 [Member] | Accident years 2018 - 2019 [Member] | Accident and Health Insurance Product Line [Member] | Short Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development		(36)
North America Commercial P&C Insurance [Member] | Short-Duration Insurance Contract, Accident Year 2020 | Property and Inland Marine [Member] | Short Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development	(25)
North America Commercial P&C Insurance [Member] | Accident Years 2019 - 2020 | Professional Malpractice Liability Insurance [Member] | Short Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development	41
North America Commercial P&C Insurance [Member] | Accident Years 2019 - 2020 | Accident and Health Insurance Product Line [Member] | Short Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development	(48)
North America Agricultural Insurance [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development	(2)	(14)
North America Agricultural Insurance [Member] | Long Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development	0	0
North America Agricultural Insurance [Member] | Short Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development	(2)	(14)
Corporate Segment [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development	9	11
Corporate Segment [Member] | Long Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development	9	11
Corporate Segment [Member] | Short Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development	0	0
Overseas General Insurance [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development	(25)	(4)
Overseas General Insurance [Member] | Long Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development	0	0
Overseas General Insurance [Member] | Short Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development	(25)	(4)
North America Personal P&C Insurance [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development		1
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
North America Personal P&C Insurance [Member] | Short Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development	(40)	1
North America Personal P&C Insurance [Member] | Short-Duration Insurance Contract, Accident Year 2020
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development	(40)
Global Reinsurance [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development	(7)	(7)
Global Reinsurance [Member] | Long Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development	0	0
Global Reinsurance [Member] | Short Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development	$ (7)	$ (7)