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, 2020	Dec. 31, 2019	Dec. 31, 2018
Equity securities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Balance- Beginning of year, assets	$ 69	$ 57	$ 44
Transfers into Level 3, assets	0	0	0
Transfers out of Level 3, assets	(3)	0	0
Change in Net Unrealized Gains (Losses) included in OCI, Assets	0	1	(2)
Net Realized Gains/Losses, Assets	1	(2)	6
Purchased, assets	23	34	37
Sales, assets	(17)	(21)	(28)
Settlements, assets	0	0	0
Balance- End of year, assets	73	69	57
Net Realized Gains/Losses Attributable to Changes in Fair Value at the Balance Sheet Date, Assets	4	(3)	(1)
Change in Net Unrealized Gains/Losses included in OCI at the Balance Sheet Date, Assets	0	0
Short-term investments
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Balance- Beginning of year, assets	6	1	0
Transfers into Level 3, assets	0	0	5
Transfers out of Level 3, assets	0	0	0
Change in Net Unrealized Gains (Losses) included in OCI, Assets	0	0	0
Net Realized Gains/Losses, Assets	1	0	0
Purchased, assets	14	6	9
Sales, assets	(2)	0	0
Settlements, assets	(12)	(1)	(13)
Balance- End of year, assets	5	6	1
Net Realized Gains/Losses Attributable to Changes in Fair Value at the Balance Sheet Date, Assets	0	0	0
Change in Net Unrealized Gains/Losses included in OCI at the Balance Sheet Date, Assets	0	0
Other investments
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Balance- Beginning of year, assets	10	11	263
Transfers into Level 3, assets	0	0	0
Transfers out of Level 3, assets	0	0	(252)
Change in Net Unrealized Gains (Losses) included in OCI, Assets	0	0	(2)
Net Realized Gains/Losses, Assets	0	0	1
Purchased, assets	0	0	50
Sales, assets	0	0	0
Settlements, assets	0	(1)	(49)
Balance- End of year, assets	10	10	11
Net Realized Gains/Losses Attributable to Changes in Fair Value at the Balance Sheet Date, Assets	0	0	1
Change in Net Unrealized Gains/Losses included in OCI at the Balance Sheet Date, Assets	0	0
Available-for-sale Securities [Member] | Foreign
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Balance- Beginning of year, assets	449	345	93
Transfers into Level 3, assets	0	11	13
Transfers out of Level 3, assets	(16)	(24)	(2)
Change in Net Unrealized Gains (Losses) included in OCI, Assets	19	13	(12)
Net Realized Gains/Losses, Assets	(1)	(1)	(3)
Purchased, assets	274	228	334
Sales, assets	(122)	(70)	(69)
Settlements, assets	(57)	(53)	(9)
Balance- End of year, assets	546	449	345
Net Realized Gains/Losses Attributable to Changes in Fair Value at the Balance Sheet Date, Assets	0	0	(1)
Change in Net Unrealized Gains/Losses included in OCI at the Balance Sheet Date, Assets	16	7
Available-for-sale Securities [Member] | Corporate securities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Balance- Beginning of year, assets	1,451	1,299	1,037
Transfers into Level 3, assets	134	23	24
Transfers out of Level 3, assets	(73)	(38)	(31)
Change in Net Unrealized Gains (Losses) included in OCI, Assets	(8)	(2)	(4)
Net Realized Gains/Losses, Assets	(30)	(4)	(5)
Purchased, assets	708	577	672
Sales, assets	(186)	(125)	(164)
Settlements, assets	(423)	(279)	(230)
Balance- End of year, assets	1,573	1,451	1,299
Net Realized Gains/Losses Attributable to Changes in Fair Value at the Balance Sheet Date, Assets	(5)	(2)	(7)
Change in Net Unrealized Gains/Losses included in OCI at the Balance Sheet Date, Assets	(6)	(8)
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	60	61	78
Transfers into Level 3, assets	0	0	1
Transfers out of Level 3, assets	(1)	(16)	(3)
Change in Net Unrealized Gains (Losses) included in OCI, Assets	0	0	0
Net Realized Gains/Losses, Assets	0	0	0
Purchased, assets	2	19	5
Sales, assets	0	(1)	0
Settlements, assets	(1)	(3)	(20)
Balance- End of year, assets	60	60	61
Net Realized Gains/Losses Attributable to Changes in Fair Value at the Balance Sheet Date, Assets	0	0	$ 0
Change in Net Unrealized Gains/Losses included in OCI at the Balance Sheet Date, Assets	$ 0	$ 0