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, 2021	Dec. 31, 2020	Dec. 31, 2019
Equity securities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Balance- Beginning of year, assets	$ 73	$ 69	$ 57
Transfers into Level 3, assets	0	0	0
Transfers out of Level 3, assets	0	(3)	0
Change in Net Unrealized Gains (Losses) included in OCI, Assets	0	0	1
Net Realized Gains/Losses, Assets	8	1	(2)
Purchased, assets	21	23	34
Sales, assets	(25)	(17)	(21)
Settlements, assets	0	0	0
Balance- End of year, assets	77	73	69
Net Realized Gains/Losses Attributable to Changes in Fair Value at the Balance Sheet Date, Assets	5	4	(3)
Change in Net Unrealized Gains/Losses included in OCI at the Balance Sheet Date, Assets	0	0	0
Short-term investments
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Balance- Beginning of year, assets	5	6	1
Transfers into Level 3, assets	0	0	0
Transfers out of Level 3, assets	0	0	0
Change in Net Unrealized Gains (Losses) included in OCI, Assets	(1)	0	0
Net Realized Gains/Losses, Assets	0	(1)	0
Purchased, assets	9	14	6
Sales, assets	0	(2)	0
Settlements, assets	(6)	(12)	(1)
Balance- End of year, assets	7	5	6
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	0
Other investments
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Balance- Beginning of year, assets	10	10	11
Transfers into Level 3, assets	0	0	0
Transfers out of Level 3, assets	(10)	0	0
Change in Net Unrealized Gains (Losses) included in OCI, Assets	0	0	0
Net Realized Gains/Losses, Assets	0	0	0
Purchased, assets	0	0	0
Sales, assets	0	0	0
Settlements, assets	0	0	(1)
Balance- End of year, assets	0	10	10
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	0
Available-for-sale Securities [Member] | Foreign
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Balance- Beginning of year, assets	546	449	345
Transfers into Level 3, assets	24	0	11
Transfers out of Level 3, assets	(11)	(16)	(24)
Change in Net Unrealized Gains (Losses) included in OCI, Assets	(30)	19	13
Net Realized Gains/Losses, Assets	(1)	(1)	(1)
Purchased, assets	275	274	228
Sales, assets	(48)	(122)	(70)
Settlements, assets	(122)	(57)	(53)
Balance- End of year, assets	633	546	449
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	(25)	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	1,573	1,451	1,299
Transfers into Level 3, assets	91	134	23
Transfers out of Level 3, assets	(76)	(73)	(38)
Change in Net Unrealized Gains (Losses) included in OCI, Assets	15	(8)	(2)
Net Realized Gains/Losses, Assets	(2)	(30)	(4)
Purchased, assets	1,154	708	577
Sales, assets	(99)	(186)	(125)
Settlements, assets	(607)	(423)	(279)
Balance- End of year, assets	2,049	1,573	1,451
Net Realized Gains/Losses Attributable to Changes in Fair Value at the Balance Sheet Date, Assets	3	(5)	(2)
Change in Net Unrealized Gains/Losses included in OCI at the Balance Sheet Date, Assets	17	(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	60	61
Transfers into Level 3, assets	0	0	0
Transfers out of Level 3, assets	(18)	(1)	(16)
Change in Net Unrealized Gains (Losses) included in OCI, Assets	0	0	0
Net Realized Gains/Losses, Assets	0	0	0
Purchased, assets	18	2	19
Sales, assets	(1)	0	(1)
Settlements, assets	(33)	(1)	(3)
Balance- End of year, assets	26	60	60
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	$ 0