Fair value measurements Fair Value Measurements (Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation) (Details) - Level 3 - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Equity securities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Balance- Beginning of year, assets	$ 44	$ 41	$ 16
Transfers into Level 3, assets	0	0	0
Change in Net Unrealized Gains (Losses) included in OCI, Assets	(2)	(1)	2
Net Realized Gains/Losses, Assets	6	2	1
Purchased, assets	37	24	27	[1]
Sales, assets	(28)	(22)	(5)
Settlements, assets	0	0	0
Balance- End of year, assets	57	44	41
Net Realized Gains/Losses Attributable to Changes in Fair Value at the Balance Sheet Date, Assets	(1)	(1)	0
Short-term investments
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Balance- Beginning of year, assets		25
Transfers into Level 3, assets	5	0	0
Transfers out of Level 3, assets			(50)
Change in Net Unrealized Gains (Losses) included in OCI, Assets	0	0	0
Net Realized Gains/Losses, Assets	0	0	0
Purchased, assets	9	16	75	[1]
Sales, assets	0	0	0
Settlements, assets	(13)	(41)	0
Balance- End of year, assets	1		25
Net Realized Gains/Losses Attributable to Changes in Fair Value at the Balance Sheet Date, Assets	0	0	0
Other investments
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Balance- Beginning of year, assets	263	225	212
Transfers into Level 3, assets	0	0	0
Transfers out of Level 3, assets	(252)
Change in Net Unrealized Gains (Losses) included in OCI, Assets	(2)	6	(2)
Net Realized Gains/Losses, Assets	1	0	1
Purchased, assets	50	56	33	[1]
Sales, assets	0	0	0
Settlements, assets	(49)	(24)	(19)
Balance- End of year, assets	11	263	225
Net Realized Gains/Losses Attributable to Changes in Fair Value at the Balance Sheet Date, Assets	1	0	1
Available-for-sale Securities [Member] | Foreign
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Balance- Beginning of year, assets	93	74	57
Transfers into Level 3, assets	13	0	9
Transfers out of Level 3, assets	(2)	(3)	(24)
Change in Net Unrealized Gains (Losses) included in OCI, Assets	(12)	3	1
Net Realized Gains/Losses, Assets	(3)	0	(6)
Purchased, assets	334	84	70	[1]
Sales, assets	(69)	(59)	(17)
Settlements, assets	(9)	(6)	(16)
Balance- End of year, assets	345	93	74
Net Realized Gains/Losses Attributable to Changes in Fair Value at the Balance Sheet Date, Assets	(1)	(1)	(5)
Available-for-sale Securities [Member] | Corporate securities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Balance- Beginning of year, assets	1,037	681	174
Transfers into Level 3, assets	24	231	53
Transfers out of Level 3, assets	(31)	(93)	(10)
Change in Net Unrealized Gains (Losses) included in OCI, Assets	(4)	(12)	15
Net Realized Gains/Losses, Assets	(5)	0	(13)
Purchased, assets	672	521	566	[1]
Sales, assets	(164)	(111)	(59)
Settlements, assets	(230)	(180)	(45)
Balance- End of year, assets	1,299	1,037	681
Net Realized Gains/Losses Attributable to Changes in Fair Value at the Balance Sheet Date, Assets	(7)	(2)	(11)
Available-for-sale Securities [Member] | Mortgage-backed securities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Balance- Beginning of year, assets	78	45	53
Transfers into Level 3, assets	1	50	0
Transfers out of Level 3, assets	(3)
Change in Net Unrealized Gains (Losses) included in OCI, Assets	0	0	(1)
Net Realized Gains/Losses, Assets	0	0	0
Purchased, assets	5	8	1	[1]
Sales, assets	0	(1)	(8)
Settlements, assets	(20)	(24)	0
Balance- End of year, assets	61	78	45
Net Realized Gains/Losses Attributable to Changes in Fair Value at the Balance Sheet Date, Assets	$ 0	$ 0	$ 0

[1] Includes acquired invested assets as a result of the Chubb Corp acquisition.