Pension Plans, Retirement Benefits and Savings Plans (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2024	Mar. 31, 2023
Pension Plans [Member]
Defined benefit plan disclosure
Service cost	$ 29	$ 27
Interest cost on benefit obligation	43	44
Expected return on plan assets	(75)	(78)
Net periodic benefit cost (benefit), amortization of unrecognized prior service benefit	0	0
Net periodic benefit cost (benefit), amortization of unrecognized net actuarial (gain) loss	2	0
Total non-service cost (benefit)	(30)	(34)
Net periodic benefit cost (benefit)	(1)	(7)
Pension Plans [Member] | Claims and claim adjustment expenses [Member]
Defined benefit plan disclosure
Service cost	11	11
Total non-service cost (benefit)	(12)	(14)
Pension Plans [Member] | General and administrative expenses [Member]
Defined benefit plan disclosure
Service cost	18	16
Total non-service cost (benefit)	(18)	(20)
Postretirement Benefit Plans [Member]
Defined benefit plan disclosure
Service cost	0	0
Interest cost on benefit obligation	1	1
Expected return on plan assets	0	0
Net periodic benefit cost (benefit), amortization of unrecognized prior service benefit	(1)	(1)
Net periodic benefit cost (benefit), amortization of unrecognized net actuarial (gain) loss	(2)	(2)
Total non-service cost (benefit)	(2)	(2)
Net periodic benefit cost (benefit)	(2)	(2)
Postretirement Benefit Plans [Member] | Claims and claim adjustment expenses [Member]
Defined benefit plan disclosure
Service cost	0	0
Total non-service cost (benefit)	(1)	(1)
Postretirement Benefit Plans [Member] | General and administrative expenses [Member]
Defined benefit plan disclosure
Service cost	0	0
Total non-service cost (benefit)	$ (1)	$ (1)