Pension Plans, Retirement Benefits and Savings Plans (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2023	Jun. 30, 2022	Jun. 30, 2023	Jun. 30, 2022
Pension Plans [Member]
Defined benefit plan disclosure
Service cost	$ 27	$ 37	$ 54	$ 73
Interest cost on benefit obligation	44	26	88	51
Expected return on plan assets	(78)	(74)	(156)	(148)
Net periodic benefit cost (benefit), amortization of unrecognized prior service benefit	0	0	0	0
Net periodic benefit cost (benefit), amortization of unrecognized net actuarial (gain) loss	0	13	0	25
Total non-service cost (benefit)	(34)	(35)	(68)	(72)
Net periodic benefit cost (benefit)	(7)	2	(14)	1
Pension Plans [Member] | Claims and claim adjustment expenses [Member]
Defined benefit plan disclosure
Service cost	11	15	22	30
Total non-service cost (benefit)	(13)	(14)	(27)	(29)
Pension Plans [Member] | General and administrative expenses [Member]
Defined benefit plan disclosure
Service cost	16	22	32	43
Total non-service cost (benefit)	(21)	(21)	(41)	(43)
Postretirement Benefit Plans [Member]
Defined benefit plan disclosure
Service cost	0	0	0	0
Interest cost on benefit obligation	1	1	2	2
Expected return on plan assets	0	0	0	0
Net periodic benefit cost (benefit), amortization of unrecognized prior service benefit	(1)	(1)	(2)	(2)
Net periodic benefit cost (benefit), amortization of unrecognized net actuarial (gain) loss	(2)	(1)	(4)	(2)
Total non-service cost (benefit)	(2)	(1)	(4)	(2)
Net periodic benefit cost (benefit)	(2)	(1)	(4)	(2)
Postretirement Benefit Plans [Member] | Claims and claim adjustment expenses [Member]
Defined benefit plan disclosure
Service cost	0	0	0	0
Total non-service cost (benefit)	(1)	(1)	(2)	(1)
Postretirement Benefit Plans [Member] | General and administrative expenses [Member]
Defined benefit plan disclosure
Service cost	0	0	0	0
Total non-service cost (benefit)	$ (1)	$ 0	$ (2)	$ (1)